Threesome Subtext

Subtextual implications in the story, about the possibility of a shared sexual relationship between three characters.

Like most other Ship Tease tropes, it might be caused by the author fooling around, with the expectation that no one will consider this a serious romantic resolution, or by teasing the fans of that particular kink to stay tuned.

Three-Way Sex is the textual version of this, where threesomes actually happen.

Twin Threesome Fantasy is a subtrope.

Compare One True Threesome, the Shipping Goggles-induced counterpart. See also Homoerotic Subtext and Incest Subtext.

Examples of Threesome Subtext include:

Anime and Manga

In Tenchi Muyo!, Ryoko x Tenchi x Ayeka, and Ayeka and Ryoko hate each other, but plenty Foe Yay occurs in universe.
Aoba/Mairu/Kururi gets a bunch of this in Durarara!!. Mairu and Kururi thank Aoba with a kiss for turning in a group of bullies; Aoba has a They Are Not My Girlfriends moment when his Blue Square members refer to them as such, but turns right around and explicitly orders them not to involve the twins in the gang wars because he's taken a particular liking to them. And then there's all the Mairu/Kururi Twincest. Worth noting that they've all kissed at some point.
Star Driver is made of this trope: implying it so freaking hard between the three protagonists (Takuto, Wako, and Sugata) while never actually resolving it.
Mahou Sensei Negima: "Sai-shou-dou-kin!?" To clarify: when one character is trying to figure out a solution to the Love Triangle she's in, her subconscious suggests a threeway.
- Going by Cosmo Entelechia, we have Nodoka/Negi/Yue, Chachamaru/Negi/Evangeline, and Fate/Shiori/Negi
In School Rumble Eri and Yakumo are both in love with Harima. There has also been quite a bit of Les Yay / Foe Yay between the two of them, culminating in a episode where they both end up dancing with each other. Before the original manga suddenly stopped, Harima was also apparently engaged to Eri while living with Yakumo, with all three apparently content with the arrangement.
In Rosario + Vampire, similar to the Negima example, Yukari Sendou is a Shipper on Deck for two people she's attracted to, because if they get together, it'll be easier for her to score a threesome.
In Death Note during the first arc: Misa-Light-L and Second arc: Misa-Light-Takada / Light-Takada-Mikami and in the prequel novel Another Note Raye Penber-Naomi Misora-Ryuuzaki
Ranka/Alto/Sheryl in Macross Frontier. At the end of the series, Alto says "You are both my wings."
Carrot/Tira/Chocolat in the Sorcerer Hunters anime. It's strongly implied the sisters would not mind sharing the man they love.
In Otome Youkai Zakuro, there is a canon Twin Threesome Fantasy with Ganryuu and twins Bonbori and Hozuki.

Comic Books

In Empowered, this is implied between the three main characters, Emp, Thug Boy and Ninjette. Emp and Thug Boy are in a very happy relationship with Ninjette as Emp's best friend. But not only are Thug Boy and Ninjette attracted to each other (he even accidently felt her up when the three of them were sharing a bed after Ninjette underwent a traumatic experience), but it's implied that Ninjette is attracted to Emp, and Emp has described herself as a "three drink bisexual." The possibility is even acknowledged during a conversation between Ninjette and The Caged Demonwolf, though Ninjette refuses to divulge her fantasies.

Film

Johnny, Lisa, and Denny from The Room

Denny: I just like to watch you guys.

Tron has to set a minor record, despite being a Disney flick. First, Alan and Lora show up at Flynn's arcade. Flynn acts a little overly familiar to them both (even if Lora's his ex), snarking that "nothing classes up the joint like a clean-cut young couple." Once they're upstairs, he casually changes his shirt in front of them, remarks Lora isn't one for small talk, and asks Alan if she still leaves clothing on the floor. The end of the scene is Lora brandishing a set of car keys and asking "Shall we dance?".
- Once Flynn's in cyberspace, there's buckets of Ho Yay when he allies himself with Ram and Tron. The scene at the Power Pool? Three pretty men in skintight, neon-lit spandex gasping and giggling over how good power from a "pure source" is.
- And after Ram dies and Flynn's found Tron and Yori? Well, see the arcade scene above. Tron and Yori are doppelgangers of Alan and Lora and in an established relationship already. Doesn't slow down any of the Ho Yay from earlier, nor does it stop Flynn from giving Yori a very passionate Last Kiss before making what he believes to be a Heroic Sacrifice to save them all.

Literature

Since Troy Denning started writing the Star Wars Expanded Universe, he's put a lot of this between Jaina, Zekk, and Jag. When Zekk and Jaina are part of a hive mind, Zekk finds Jag attractive. Then in Invincible, after one fight, Jaina invites both of them to bed with her at the same time.

Live-Action TV

Doctor Who: Rory/Amy/The Doctor have a lot. Amy has on occasion called them her "boys," and they don't disagree. And earlier on, there's a bit of Jack, Rose and The Doctor.
- In the latter case, all three kiss over the course of one episode.
- In fact, Jack is canonically attracted to both Rose and the Doctor, and Rose to both Jack and the Doctor.
Glee: In the episode "Prom Queen" Rachel and Mercedes both ask Sam to the prom together. They go as a group of three (and are later joined by Jesse).
In the second season of Buffy the Vampire Slayer, Angelus/Spike/Drusilla is all but canon. Both the guys lust after her but also have massive Foe Yay with eachother. There is that infamous line in later seasons about Angelus and Spike having sex "that one time" (confirmed to have happened by Joss Whedon himself) which probably happened when Spike was still with Dru. And, of course, when Angelus first shows up (after being de-souled) he kisses Spike passionately on the forehead, prompting Drusilla to gleefully squeal "We're a family again!" Dude, was that even supposed to be subtext?
The joyful Abby/Becker/Connor reunion hug in the Primeval series 4 opener is bookended near the end of the series 5 finale: Our heroes walking off into the sunset: Matt/Emily in their own tight little pair, followed by our trio: Abby with an arm around Becker's waist, and holding hands with Connor. Becker also has an arm around Abby, and at the last second, he reaches over to put it around Connor, too. Awwww. Made all the more amusing by the fact that Becker once refused to answer a direct question about his orientation.
Happens a lot in How I Met Your Mother, in almost every permutation of the the five main characters, but most often and explicitly Ted/Marshall/Lily, where Ted is treated like an extra spouse in many, many gags. Marshall and Lily even once say "If we get married, we have to marry Ted!", when Lily gets pregnant, Ted keeps referring to it as "our" baby, and a montage in one episodes features Ted as the third factor in various famous couples (eg. Lady and the Tramp and the Bowl Of Spaghetti).
Happens between Cory, Shawn and Topanga on Boy Meets World:

Shawn: I think the three of us could be very happy together.

In one Frasier episode, a case of misinterpretation leads Roz to believe that Niles and Daphne are asking her to have a threesome with them. The ensuing conversation is hilarious, and the fact that Roz doesn't seem too averse to the idea cause Niles and Daphne to mercilessly tease her after the miscommunication is cleared up.

Video Games

Skies of Arcadia has so much of this between Vyse, Aika and Fina in-universe that Ship-to-Ship Combat among the three of them in fandom is nearly nonexistent.
- One of their boss fight opponents remarks on it!
Laharl, Etna and Flonne in Disgaea. Each one has incredible chemistry with the other two. It's so obvious, it's painful.
Ar tonelico series lets you double date (and later triple date) the Reyvateil heroines in your party. While you still can settle with only one girl in the end, almost no one in the game's going to call the protagonists out for diving into multiple girls' minds at once. The subject is even more awkward since "diving" is given the same intimacy level as sex.
In Katawa Shoujo, all of the heroines come in pairs of friends, so there's plenty of potential for this. The high point is probably Hanako and Lilly's routes, as the two are extremely close Heterosexual Life Partners and both of their routes feature the other to a great extent. Befriending the other is actually required to get either of them, and their routes in Act 1 don't diverge until the very end. It's repeatedly made obvious how much closer than normal friends they are, with Hanako even saying that she loves Lilly in her route. To cap it all off, at one point in Lilly's route during one of their pyjama party/tea party get-togethers, Akira mentions that plenty of guys would be very pleased to see Lilly in such skimpy clothing. As they're already together, Hisao privately agrees. Akira goes on to add that he even gets to see cute Hanako in her nightie as well. Hisao deflects, but Hanako haltingly says that if it's Hisao, she doesn't mind. She doesn't seem to realise what she's implying, but everyone else sure does, and quickly change the subject.
- Early in Emi's route, Misha teases Hisao about having lunch with both Emi and Rin, and mentions that the three of them are cute together.
- Shizune's route also features Misha very heavily, and Misha canonically is in love with Shizune, though it isn't requited. In fact, the route arguably focuses more on the friendship between the three of them than on Hisao and Shizune's relationship specifically.
Kingdom Hearts: Sora/Riku/Kairi. Sora/Kairi is (probably) the Official Couple and the first KH game strongly implied that Riku also had a crush on Kairi (jury's out on whether he still feels this way about her), while Sora/Riku spend a lot of Kingdom Hearts 2 and Dream Drop Distance being pretty darn hoyaytastic to each other. When it comes to the dynamics of the three of them together, Kairi even comments to Riku in Kingdom Hearts 2 that Sora is completely hopeless without them and holds Sora and Riku's hands between her own in order to let Sora see that what looks like Ansem Seeker of Darkness is, in fact, actually Riku. The opening FMV of Kingdom Hearts 2 even briefly shows a scene of the three of them lying on a beach together, holding hands.
Fire Emblem: Genealogy of the Holy War, possibly. Celice and Yuria will rule Bahara together...along with whoever you paired Celice with (with the exception of Tinny unless you pair Tiltyu with a common dude)). The most obvious example would be Celice/Rana/Yuria; Rana and Yuria are quite close and in her lover convo with Celice in the final chapter, she tells him she loves him but feels terrible about Yuria, implying she both ships Celice/Yuria and has special feelings for the girl herself.

Web Comics

In Drowtales:
- Snadhya x Mel x Zhor is cannon, but Snadhya and Zhor are not romantic toward each other, they both know about the other side of the triangle and are happy for Mel. It's suggested Quain and Ash are involved in a similar relationship.
- Fourth Wall Observer Kiel ships herself with both Nau ("Remember that I bid on you first when you're a tall handsome prince.") and Naal ("Oh and add a princess to the lot just for fun's sake!"). Naal doesn't seem to mind, Nau objects to Kiel describing him as "manly and tiny".
Girl Genius: Agatha/Gil/Tarvek, outright suggested by Moloch, Zeetha, Lucrezia, the Castle, Agatha's Animated Actor in a side story, the trio themselves on an opiate high, and most recently by Gil and Tarvek themselves, perfectly sober and in canon.
In Schlock Mercenary, uplifted ape McGruger explains to Nick that eating breakfast termites out of a girl's hair is romantic ("Hey, don't look at me like it's my fault. The uplift congress left that bit in."). Then Nick find a termite in Kathryn's hair.

McGruger: Don't look at me, Nick. You groomed her, you get to eat the bug. If I eat it then things start to get a little kinky.