Rather than requiring someone to pick one person out of several potential love interests, or have a Your Cheating Heart situation, all parties involved agree to be together and share cooperatively. This typically lacks any drama of the decision itself, but has a different form of drama in how people make arrangements with each other, manage jealousy and households, and deal with the disapproval of society as a whole.
Individuals who identify as polyamorous may consider themselves "Ethical Sluts". Not all ethical sluts are polyamorous, and not all people in polyamorous relationships have the "ethical slut" attitude. The difference between a polyamorist and a swinger is that a polyamorous person has broad-spectrum involvement in their partners' lives. Swingers are largely there for the sex, often for the novelty of an unfamiliar partner.
Compare Marry Them All. Contrast No Romantic Resolution. The Love Triangle or Love Dodecahedron can develop into this. When a foreign or alien culture is particularly accepting of this, you may get Exotic Extended Marriage. See Threesome Subtext for when this is hinted in canon, and One True Threesome for Shipping examples. Not to be confused with Polyarmory.
May develop into a case of the Supporting Harem if one partner receives preferential treatment over the others.
For more details and some Real Life tidbits, visit Useful Notes On Polyamory. Truth in Television, obviously, though how culturally accepted it is varies wildly depending on the place, time period, and social standing. Since poly is slowly entering the mainstream, there is an emerging trope of No Polyamory; fiction where the characters could/should be aware of the idea, but act like it's inconceivable.
Cross Marian he has many lovers. We met one—Anita, and heard of another, which was her mother. Both seemed to love him, and want to help him. They support the Dark Order just to do something for him. he is apparently a good lover, and he seems chivalrous in general when it comes to women. He hates the Order, but stayed because Lenalee asked him to, and earlier he protected Lenalee, and held her, on the crumbling ark.. His character profile by Hoshino states he likes "good women." So, apparently, he cares for his lovers outside of sex, otherwise, whether, or not, they're good women probably wouldn't matter. And he isn't unethical about it, as his dislikes in the profile are "dirty bastards." Allen also states that Cross lives with, and off of lovers in different places. So, apparently, it's not just Anita and her mother that care enough to do things for him. And, Anita and her mom were prostitutes, so they certainly did sleep with other men. Apparently, Cross does not care where his women come from, or who they sleep with—just that they're good women.
Done with after the credits in Kanokon's Gecko Ending — Chizuru wins, and Nozomu is Put on a Bus... But wait, she returns after the credits and they decide to share Kouta. Which kinda sucks for him, as both of them are extremely oversexed yōkai (Japanese demons), and he's mostly Asexual. Of course, he doesn't get a say in the matter.
It's also done part way through the manga, where after Kouta tells Nozomu he loves Chizuru, she leaves... the immediately comes back and declares herself his lover. Chizuru's reaction is basically "Gah, whatever, I'm still number one." However, while Kouta's opinion is not asked initially (like the anime) he later declares he loves them both and is going to be faithful to them both and no one else.
Her Royal Majesty Iono from Iono the Fanatics somehow manages to be in a romantic relationship with an entire kingdom worth ofSchoolgirl Lesbians. And satisfy them all. Unless, of course, she is traveling abroad, in which case her paramours start a civil war, with one fraction believing in her eventual return and the other striving to bring her back by force.
The last chapter of To Love-Ru hints that this might be a viable option for Rito; at the very least, Lala is enthusiastic about to the point that it seems to be her preferred option. In the sequel, To Love-Ru Darkness, one of the girls tries to open up this route (Lala approves), and is only hindered by Rito's belief in monogamy.
In Mahou Sensei Negima!, it turns out that Nodoka views this as an acceptable solution to the love triangle between herself, Yue, and Negi, showing up in her ideal world. Luna aka Shiori's own ideal world shows that she wouldn't mind having both Negi and Fate.
Marmalade Boy: Jin and Rumi Koishikawa, and Youji and Chiyako Matsura (later Jin and Chiyako Koishikawa, and Youji and Rumi Matsura) seem to hint at this. Swapping spouses is one thing; all moving into the same house together and spending all their free time together.... awfully suggestive.
Tenchi Muyo!: Seina slips into this trope at the end via a Marry Them All resolution, though it still has the feel of the former, and Word of God says Tenchi himself fits this trope. In both cases, it helps that they're in line for the throne of Jurai (Tenchi being the Crown Prince), a position that, as far as we can see, traditionally comes with multiple wives (the current one has two). It also helps that the Jurian equivalent of the Queen Mother fancies herself a matchmaker, is a master of The Plan, and seems to prefer this outcome.
Girlfriend 1: Why are we dating such a playboy again?
Girlfriend 2: Because it's one of our weird hobbies.
Sayonara, Zetsubou-Sensei has a rather complicated example of this. Nozomu loves the First Girl, Fuura Kafuka. However, it's revealed in the final chapters that she was Dead All Along. Her organs, however, went to the girls in Nozomu's class, causing him to see her in his students one at a time. (As his sister Rin puts it, he's in love with her corneas-in Abiru-then her lungs-in Komori-then her heart-in Chiri.) On paper, he's in an endless cycle of marriage and divorce between the girls. In Nozomu's eyes, he's married to Kafuka and Kafuka alone. To outsiders, it looks like this trope, especially since all the girls have children by him (that eerily look like Kafuka).
High School DxD is based around this trope. Initially set up as a traditional Harem Series, it deviates from the norm because none of the girls care that the others like him too. In fact, several of them actively help the others get closer to Ise, culminating in the rest of the harem helping the Ise and Rias become an Official Couple. At least two of them stated they were waiting for this to happen before they went any further with Ise themselves. And for her part, Rias has no problem with polygamy as long as she's number one.
Used in Anatolia Story, crossing with Twin Threesome Fantasy. Yuri's handmaidens Ryuhi and Shala both like Kail's advisor Kikkuri, and they bed him separately. They both get pregnant. Kikkuri had believed he was sleeping with one of the girls, but instead of "choosing" he marries them both. At the end of the story, the girls have given birth to... twins! (Ryuhi has two boys, Shala has two girls). They all live more or less happily under the same rooftop.
The High School AU manga of Tengen Toppa Gurren Lagann features Simon/Nia/Dark Nia in its ending (Dark Nia in this version is a secondary persona/imaginary friend that Nia created from being denied any friends by her overprotective father).
ElfQuest. Many of the elves end up 'lifemating' with two others, and their relationships are generally loving and long-lasting (and in at least three cases, explicitly mentioned as a sane alternative to jealousy and rivalry). Problems tend to arise only with Recognition, a form of 'love'/lust based entirely on involuntary instinct. Examples: Leetah/Cutter/Skywise, Skot/Krim/Pike, and Dewshine/Scouter/Tyleet.
Strangers in Paradise: Francine, David, Katchoo, Casey, Freddie and even Tambi all alternate between being friends, enemies, sexual partners and various places in between, and also rotate living with each other as one or more of those options. After David reveals to Casey and Katchoo that he is dying, they all (well, most) decide for Katchoo, Casey and David to form a sexual relationship in order to produce a child and because they all do love each other, even though Katchoo only really loves Francine, David only really loves Katchoo, and who Casey really loves is more complicated than even the reader thinks at the time. Once David dies the series settles down into Katchoo-Francine and Casey-Tambi (!?!?!?!?!?), though even then David sticks with both pairs in spirit and via his child with Katchoo.
A DC Elseworlds story, "Superman/Wonder Woman: Whom Gods Destroy", written by Chris Claremont, is set in a world where Superman made his first public appearance in the 1930s, when he met Lois Lane. Cut to the present day, where Lois Lane and Lana Lang are in their seventies and are very close friends. Despite Superman's presence during WWII, the Nazis are still in power in Germany, due in part to the interference of Ares and several other Greek Gods. Another faction of Greek Gods, led by Artemis and Athena, grant Lois and Lana eternal youth and superpowers (Lana becomes the Oracle of Delphi and Lois becomes Wonder Woman), and they team with Superman to defeat Ares' faction. Part of Ares' plan is to separate Clark from Lois and Lana, since their powers somehow bind them together. When Lana explains to Superman, "Ares doesn't want the two of you together," (referring to Clark and Lois), Hecate, the Greek witch-goddess, corrects her, "No, the three of you." By the end of the four-issue arc, Clark, Lois and Lana are happily living together in his Fortress of Solitude (the now young Lois changed her name to Lois Lang) in a polyamorous relationship.
At the very end of Superman: Red Son (an Elseworld), when Lex Luthor uses his genius to turn Earth into a utopia, it's said that the "triple" replaces the "couple" in human relationships.
In the last issue of the Secret Six ongoing, Scandal Savage decides she can't choose between her girlfriends and proposes to both of them at once.
In League of Extraordinary Gentlemen Century, Mina, Allan, and Orlando are in a somewhat complicated polyamorous relationship. It's complicated because Orlando's a Gender Bender, and Mina doesn't seem to enjoy his company so much when he's male.
In Demon Knights, Jason Blood and Etrigan are two beings who were magically joined together by Merlin, and who are each married to Madame Xanadu. It sort of works because only one or the other can inhabit the human plane of existence at a given time; the one who isn't on the human plane ends up in Hell during that time.
Poly seems to be a normal state of affairs in the universe of On A Clear Day You Can See Forever; that doesn't stop there from being issues of jealousy and insecurity, however. Mamoru is not terribly happy about Usagi's affections for her senshi, especially Rei, but he also knows that he has no right to deny them. For poor Ranma, from another world entirely, the concept of sharing is even harder to deal with.
The Epilogue of I Am What I Am has this, but with a twist. Xander (who has the memories of his entire life up to his death which he has been using to alter and improve things) is now with Buffy AND Faith and it's heavily implied that all three are sharing the other two (no sex yet, though).
In Kyon Big Damn Hero, Kyon's uncle Keiichi arranges this with his aunts. Also, Haruhi is working towards this with Kyon and all females of SOS Brigade.
In Time Braid, Sakura, Hinata and Naruto eventually get to the point where all three are romantically involved, though Sakura does state that it takes effort to make their relationship work.
In Strangetown, Here We Come, the main trio eventually end up like this with Ripp and Johnny both going out with Ophelia, and before the fic died it was implied that Johnny and Ripp might start going out as well - Ripp was canonically interested, and Johnny at least had a whole lot of Ho Yay on his side.
Hands: The Love Triangle between Andrew, Twilight, and Fluttershy eventually evolves into this, as the girls are willing to share Andrew if it means not fighting over him anymore.
In the first chapter of the Doctor Who fanfic "Come And Knock On Our Door" by Maribor (the title is an homage to Three's Company), Amy attempts to seduce the Doctor. When he protests that she's married, she answers, "We talked. He's on board." The story, along with the sequel "Man About the House" explore the Doctor, Amy and Rory in a polyamorous relationship and impending parenthood (the Doctor is able to help Amy conceive despite her inability to have children, as mentioned in "Asylum of the Daleks").
Maribor's "Epistolary", which follows Amy and Rory's lives after they were stranded in the past after "The Angels Take Manhattan", also implies an 11/Amy/Rory relationship existed in flashback scenes.
In the Frozen fanfic Sorry About The Mess, this is how the trolls view marriage and relationships in general, with wives having multiple husbands while any one of those husbands may have multiple other wives as well. Kristoff says that for a time he had five fathers and three mothers.
All residents of the Avengers Tower being in a happy poly relationship seems to be popular in fandom.
Bandits has both the main characters falling for their hostage turned ally. When she is pressed to chose she deliberately chooses "not to chose," and is last seen happily with both of them in Mexico.
Les Chansons d'Amour: Julie and Ismaël resolve Ismaël's attraction to his coworker Alice by forming a ménage à trois which doesn't survive the first act: when Julie dies, Alice and Ismaël go their separate ways.
Jules et Jim focuses on a three-way pairing, with an eye to determining whether it can possibly last. In and of itself, polyamory is deemed acceptable, but "pioneers must be humble and unselfish," and jealousy results in a murder-suicide.
Another one from Woody Allen. When Melodie's disapproving mother Marietta shows up in Whatever Works,she goes on to date two men that agree to share her.
A woman and two men try this in Paint Your Wagon. It sort of works for a while; one of the men eventually departs, not so much because of the triangle, but that the area's becoming too civilized for his taste.
In Scott Pilgrim vs. The World, Wallace Wells, Scott's gay roomate sleeps with Stacey Pilgrim's boyfriend Jimmy and another nameless guy later on. His boyfriend, 'Other Scott' does not seem to mind as at one point they're all seen sleeping in the same bed together, as well as Scott.
The play and film Design For Living revolves around the three-way relationship between two (male) best friends who are both in love with the same woman. They try this, but jealousy breaks them up and the woman marries someone else... until she decides that she still loves her boys, and as they want her back, the three of them decide to give it another try.
In the 1999 TV movie Journey to the Center of the Earth, after arriving to the land beneath the crust, the adventurers encounter a village of humans. Their guide McNiff immediately takes a liking to the local women, who respond to the big man from above. He then sees men glaring at him and finds our they're the women's husbands. Apparently, they practice polygamy. He grins and takes two women to a shack to exchange cultural knowledge. The idea of limiting relationships to only two people seems ridiculous to the locals, one woman asking in disbelief if women from above really like monogamy.
The Dutch movie Liever Verliefd ("Preferably In Love") has the main character marrying both of her love interests at the end.
Robert A. Heinlein's works feature this heavily, especially as he got farther into his career, which gives it strong overtones of Author Appeal, even though he never commented on whether or not he and his wife Virginia were exclusive. Most of the works in question advance the concept that a truly free individual should not be restricted by cultural taboos in choosing whom or how many to love.
The Moon Is a Harsh Mistress features protypical matriarchal group marriages - that is, the woman is the dominant partner and has full discretion as to whom or how many men she marries.
Stranger in a Strange Land has Mike, a human with Martian values, form an entire quasi-cult around the notion of Free Love. Said novel was widely influential in the counterculture of the American 1960s and arguably predicted it.
Friday starts with the titular protagonist in a group marriage in New Zealand, although they divorce her after she exposes their racist hypocrisy. She later joins a much healthier group marriage.
Time Enough for Love and all subsequent novels in his "Future History" series feature Lazarus Long's increasingly extended family, designed as a group marriage in which individuals are free to come and go as they please but all are mutually responsible for the maintenance of the household and care of the children. The only restriction on who sleeps with whom is genetic compatibility, and not even then if there is no risk of pregnancy.
Most of the main cast of Diane Duane's Door Into... series. Including the dragon. And the FIRE ELEMENTAL.
Gail Dayton's One Rose Trilogy takes place in a realm where group marriage is the norm.
In Realm of the Elderlings, Fool told Fitz that he had two fathers and one mother, which was quite common in his land.
The House of Night has many (sometimes with more than three people) involving Zoey, usually some sort of combination out of Zoey/Erik/Heath/Stark (and sometimes Loren). Zoey/Heath/Stark was a thing for a while.
The main character Rand al'Thor from the Wheel of Time series, falls in love with three women, who in turn all fall in love with him. Leads to a fair bit of angst until the women decide to share, and put it to Rand as a fait accompli. Slightly averted, as one of the women grew up in a culture where this is common unheard of; however, her culture does accept the (still rare) arrangement whereby a man can marry two women who are de facto married to each other as well.
The hero of With a Single Spell by Lawrence Watt-Evans falls in love and gets married during his quest to win a princess. The king is still insistent about reducing the headcount at his castle, and his wife is very practical.
Lizzie ends up with both Beardsley twins in the Outlander series.
Some of Octavia Butler's novels involve people ending up in this sort of situation, usually because of Bizarre Alien Biology — which the humans may struggle against before finally giving in. In the series Lilith's Brood a family may involve a human male, a human female, an Oankali male, an Oankali female, and an ooloi. In Fledgling, each vampire keeps a small collection of humans as a family (and a food source).
In Wen Spencer's A Brother's Price, a plague has left the world seriously gender-skewed, with 5-10 girls born for every boy. The solution for this is for boys to marry all sisters in a family. The hero, of course, goes on on to marry all of the princesses of the realm.
In Isabel Allende's Eva Luna, male lead Rolf Carlé gets into a relationship like this with two girls that not only are both his lovers, but his cousins as well. It lasts some years, until Rolf leaves the "colony" they live in and goes to the city; there's no ultra dramatic break-up, and the cousins are later Happily Married to local males whereas Rolf becomes an Intrepid Reporter and gets involved with Eva, the female lead.
This is the eventual solution when Honor's Second Love turns out to be already married. As mentioned above, it takes some pretty extraordinary people to make this kind of thing work, and fortunately they all are: the man is her former boss, Hamish Alexander, the Earl of White Haven, whose wife Emily is a triplegic who Can't Have Sex, Ever. The whole thing is her idea more than anybody else's, and there's enough Les Yay between her and Honor to suggest that, for them, the marriage is not merely a formality. In the meanwhile, though, there's a lot of wangsting, due to the existing marriage and resulting adultery; not to mention the media circus and political maelstrom that would ensue since all three characters are celebrities.
Grayson already practices Exotic Extended Marriage (polygyny, as women outnumber men on the planet by three to one), but it's hinted (by Allison Harrington, Honor's mother) that at least some of the women are bisexual, and that, if not openly encouraged, it is at least quietly tolerated.
Leslie, Niall and Irial are all totally cool with their threeway -but not threesome- relationship.
To a lesser extent, Ani and Rae seem fine with sharing Devlin as well.
In The Sharing Knife, polyamory is rare, not surprising given the pre-Industrial setting. However, it's not unheard of among the Lakewalkers. When one of the Otter women realizes her husband is sterile, she is urged to divorce him and take another who can give her children. Out of love for him, she refuses. Instead, she chooses a second husband, and all three of them marry each other. Among the Lakewalkers, marriages are proven by magical wedding bracelets. As each of the threesome has two bracelets proving that they are simultaneously married to two different people, there is nothing anyone else can do. The clan eventually gives up trying to break up the threeway marriage, and the children are considered the offspring of all three parents.
This seems to be pretty common practice in Kushiels Legacy wherein many people have multiple sexual partners with whom they are in love.
Older Than Print: In the eighth story of the eighth day of Boccaccio's Decameron, "Two men keep with one another: the one lies with the other's wife: the other, being ware thereof, manages with the aid of his wife to have the one locked in a chest, upon which he then lies with the wife of him that is locked therein." At the end, they call it even and, decide, along with their wives, to live so that "each of the ladies had two husbands, and each of the husbands two wives."
In Jennifer Crusie's Crazy For You, the female protagonist's mother is involved with a best friend as well as a husband.
In Sewer, Gas & Electric, the eco-pirate Dufresne is in a polyamorous relationship with an online journalist and her male-model second boyfriend, complete with threesomes. The model is bisexual and has no problem with this, but Dufresne's own lingering insecurities from an Amish upbringing compel him to wrestle his co-husband into submission each time they meet, offsetting his doubts about his own masculinity.
This is the resolution to the love triangle between Eric, Beth, and Korendil in Mercedes Lackey's Urban FantasyA Knight of Ghosts and Shadows. The relationship continues happily throughout the next book, but Eric breaks up with them amicably between books two and three.
In the sci-fi Uplift series, some alien species require more than two individuals all mating together in order to reproduce. Among the species with this variety of Bizarre Alien Biology are the Gubru (require one female, one blue male, and one yellow male) and the Brma (require an alpha female, a beta female, an alpha male, and a beta male). It's also becoming popular among humans, possibly because of our uplift of the by no mean monogamous chimps and dolphins. Robert's mother has four husbands for instance.
In Existence it's briefly mentioned that Hacker has two girlfriends and Gerald is in a group marriage.
Vonda McIntyre seems to like this: In the Starfarers series, there's at least one married triad (who were previously a tetrad, and almost became one again, but in both cases someone died); in Dreamsnake, it's monogamous characters like Arevin who stand out.
The sci-fi novel Forget Yourself features polyamory as a major theme, though it is never mentioned by name.
In A Song of Ice and Fire, Aegon the Conqueror was married to both of his sisters; each of them rode one of the three famous Targaryen dragons. It's been suggested to Daenerys that she should have two husbands likewise.
At the end of Michelle Cooper's Fitz Osborne trilogy Toby and Julia have a loving but nonromantic marriage, and both are involved with Simon. This proves to be an ideal solution for them, as it allows Toby to maintain his relationship with Simon while also fulfilling his duties as king; Simon to father the heirs to the throne of Montmaray (to which he was the rightful, but illegitimate heir; yes, he and Toby found out they were secret cousins after they were sleeping together)note Toby's sister Sophie sleeps with Simon after learning that they're secret cousins, so it's not really that big a concern in the book; and Julia to marry on her own terms. The three of them live happily together with their children.
In The Millennium Trilogy, there is a situation somewhere between this and Friends with Benefits; Mikhael Blomkvist and Erika Berger's inability to stop sleeping with each other led to the failure of his marriage, but Erika's husband doesn't mind her seeing Mikhael, and the two men get on well together. Erika and her husband have experimented with threesomes (though only with other people since Mikhael himself wasn't interested,) and they don't actually live together, but otherwise it works quite similarly to this trope.
Occurs many places throughout The Bible, making this Older Than Feudalism at least. Many important figures had multiple wives, such as Jacob in the Book of Genesis who marries sisters Leah and Rachel (although the former was thanks to a Bed Trick). In the Books of Kings, David had eight wives and several concubines, in common with most kings of the time. The champion, however, is probably Solomon, who takes it Up to Eleven with 700 wives and 300 concubines! The practice had largely died out by the end of the Old Testament, and the New Testament averts it by teaching more or less straightforwardly that a man should only have one wife.
The Color Of Distance has Juna mention her failed "group marriage". In the sequel we see that her brother is in a similar marriage and they are commonplace. Often two people are monogamous within the marriage, but in passing a triad consisting of two brothers and a woman is mentioned. The real purpose in group marriages is a sense of family and community, and sharing child rights.
Mercedes Lackey's Bedlam's Bard ends with two humans and an elf as a happy triad. In the next book they're still living together, but the book after that starts with the human man having split off amicably.
Dr. Phlox of Star Trek: Enterprise comes from a poly civilization. He has three wives, each of whom has three husbands, who themselves has three wives each. In a way you can say that all the Denobulan are part of one giant marriage group. There's also indication that three ISN'T the limit, since Feezal, who already had three husbands, attempted to seduce Trip. Phlox felt that Trip should have gone for it, indicating that Denobulans also lack any concerns about marital fidelity.
At least one novel has indicated that group marriage is legal on Earth. (At least two of which were written by Vonda McIntyre.)
On DS9, there's a throwaway reference to a Bolian officer having a wife and a co-husband.
In the Next Generation episode "Data's Day", Data mentions that four-person family units are the norm among Andorians. It should be noted that this is also because, according to expanded works, Andorians have four sexes.
Big Love, being based around Fundamentalist Mormon Polygyny is both an example and a counterexample to this trope, especially since it's depicted that women are often wed as young teens, ergo matches are made outside of the realm of informed adult consent. However, the family of the main characters mostly came together as consenting adults, though Margene revealed in season 5 that she was only 16 when she married Bill. The age of consent in Utah is 18.
Sister Clarice Willow on Caprica is married to multiple men and women, who are all married to each other. This is implied to be unusual but perfectly legal.
Londo Mollari on Babylon 5 has three wives and a mistress, but it's kind of an aversion since he would gladly toss the polyamory and his three wives out the window, and marry just the mistress he truly loves (his marriages were political ). He sums it up rather bluntly:
Londo: Here. Look. These are my three wives: Pestilence, Famine, and Death. Do you think I married them for their personalities? Their personalities could shatter entire planets! Arranged marriages. Every one. But they worked out, they inspired me! Knowing that they were waiting at home for me is what keeps me here — 75 light-years away!
Phoebe finds out at the end of the third season that the mother who raised her isn't her biological mother; when she meets her biomom in Montauk, she tells Phoebe that herself and her parents were "kind of a couple" together.
In an early arc Chandler is in a relationship with a woman who has two other boyfriends, he finally cuts it off when she gains a fourth.
Mac on JAG has to defend a polygamist in "Killer Instinct".
One of the victims of the week in Pushing Daisies has four wives. The heroes are pissed about this—not for any moral reason, but because it makes "My wife killed me" a whole lot more complicated than it needs to be.
In The Tribe, Ram made wives out of sisters Java and Siva, mostly to show off his own power. When he meets Ebony (the third sister) he want to marry her too, to "complete the set", and she seem receptive to this at first. When she falls in love with Jay instead he becomes heartbroken for her betrayal.
In a Sliders episode, the characters end up in a world where the bikers are raiding oil trucks to convert it to gas for their bikes. Apparently, all cars run on alternative fuels here. Mallory (not Q-ball) hooks up with a woman who turns out to be the leader of the local biker gang. One day, he enters her place to find her in bed with two guys (all still clothed, for PG reasons), who she calmly explains are her husbands. Furthermore, she invites him to join them. He refuses and later finds out that his friends knew that polygamy is legal and perfectly acceptable in this world.
Subverted in American Horror Story: Asylum with Kit, Alma and Grace. They set up housekeeping together but it does not end well, especially for Grace.
Suggested in Too Many Cooks, with Darren and Claire in an embrace being approached by T.L. and Sasha, and immediately turning around and kissing them.
Vous étiez ma vie comme la nuit et le jour Vous deux, nouez, filiez mon parfait amour Un matin vous m’avez condamnée à choisir Je ne vous aimais qu'à deux Je vous laisse, adieu Choisir serait nous trahir
"Millionaire Girlfriend" by Jonathan Coulton seems to suggest this, since it will be "my beloved and my millionaire girlfriend and I", until the last verse where it's revealed that he's just working to get power of attorney over all her assets.
The Noël Coward play Design for Living explores every possible combination of playwright Leo, artist Otto, and decorator Gilda before finally having them inform her husband Ernest that his services are no longer required, as the three of them intend to live in what Ernest decries as a "disgusting three-sided erotic hodgepodge."
In Gilbert and Sullivan's Trial By Jury, someone proposes that the defendant (in a Breach of Promise to Marry suit) "marry this lady today, and marry the other tomorrow". That's considered a satisfactory solution until one of the attorneys points out to the judge that such an arrangement would be illegal.
There are fan mods that let you have a relationship with more than one Love Interest in Dragon Age: Origins (one of them is outright called Polyamory). If you then marry Alistair to Anora and make him sleep with Morrigan or marry her yourself love polygon gets rather complicated.
You can try it in Star Wars: The Old Republic, particularly with a male Sith Warrior that's corrupted and romancing Jaesa, but is still nice to Vette. None of your potential love interests will go for it. Though any romance option with the Imperial Agent and Vector is going to be a crowd scene since he's tied to a Kilik hive.
Overlord II where you collect three mistresses over the course of the game, are allowed to keep them, and chose any one to be your primary one at any point in the game. It's possible to get all of them to "get along" with and without innuendo.
One of the (very hard to get) endings in School Days has Sekai and Kotonoha do this to the protagonist Itou Makoto.
Angel Starr references having multiple boyfriends in Ace Attorney, even telling the judge that there's a slot open for him.
In the final route of Duel Savior Destinynote And also the original Justice, though much more blatantly this seems to be the final solution to the romance. Crea takes center stage, though.
Alexstraza from the World of Warcraft, though widowed many times over, used to have several husband-equivalents at once.
The English Dating SimSummer Session features a three-person relationship as one of the canon endings; indeed it's impossible to get that particular girl without sharing her with your roommate. It's unclear whether the two male characters are actually involved with each other or just sharing the girl in a V.
Phantasmagoria 2 has bisexual hero Curtis wishing he could run off and live with his girlfriend Jocilyn and his doomed gay coworker Trevor, both of whom are in love with him. At one point Curtis was dating both Jocilyn and his BSDM loving coworker Therese, but only Therese was okay with the idea of their having an open relationship.
Quest for Glory V: A glitch lets you propose to three love interests and no one objects. Though only one appears as the Hero's consort at the end.
Type-Moon: Really loves this option in their games. In Tsukihime in Ciel's Good Ending Arcueid and Ciel agree to "share" Shiki between each other, after their great rivalry during route. Even more significant is this in Fate/stay night in Unlimited Blade Works route where rather than a classic simple romance we get love triad between Shirou, Rin and Saber, in which heroes gradually develops feelings between each other, have a shared date and eventually their relationship is completely fulfilled in route's Good End where they ends live together as mutual lovers.
Is even hinted that Rin has bisexual tendencies, so they are total Type 8 triangle.
Fans! has Rikk, Alisin and Rumy become a triple. This arc continued with the marriage surviving some storms to the point where thanks to alien medtech Alisin is currently carrying a baby Rumy and Rikk conceived.
Angela of Punch an' Pie believes in polyamory, but isn't successful in practicing it.
The Flash Forward strips in the Space Arc of Arthur, King of Time and Space suggest that the Arthur/Guenevere/Lancelot triangle resolves this way in at least one timeline. Other strips hint that Arthur is well aware of the affair between his wife and his best friend in at least some of the other timelines, as well, but studiously ignores it.
Girl Genius: The Heterodyne Castle seems to prefer this as the ultimate solution for the Love Triangle between Agatha, Gil, and Tarvek. Comments by the Castle indicate that this was likely a common practice for previous Heterodynes. The Ho Yay/Foe Yay between Gil and Tarvek doesn't really do much to rule out such a solution. It's teased in some non-canon spin-offs, too.
Karin-dou 4koma: Ran/Shizuki/Tamaryu, as per Ran and Shizuki's decision to resolve their rivalry over Tamaryu by having Shizuki be Tamaryu's wife, Tamaryu be Ran's wife, and Ran be Shizuki's wife. Ran explicitly says that the three of them are together a monogamous couple. Although Tamaryu isn't happy about the romantic aspect, they do get along quite well. When Eldelita attends a sleepover with them, she feels as awkward as someone invited to play and stay overnight at a newlywed's house.
Cats N' Cameras: Main characters, Jim, Josephine and Sari are in what appears to be casual threesome. Jim and Jo are an established couple. Sari seems to be Friends with Benefits to both of them.
The Insecticomics has a lot of this, usually in the form of open relationships.
In the backstory of S.S.D.D Richard briefly attempted a threeway with the lesbian couple he roomed with in college, it didn't work out.
Family, an Internet television show, features a polyamorous triad (V formation).
Mackenzie in Tales Of MU falls into this situation pretty quickly. As things stand, she is the lover of Ian, Steff, and Amaranth, the former two also being Amaranth's lovers (but not with each other as well).
Metamor City has the Psi Collective members live in families/cells, with (preferably) one male and several females. This is because they believe that a war with the "mundies" (unpowered humans) is inevitable and that they'll need to prepare by breeding as many psychics as possible, but there aren't many powerful males. At the end of Making the Cut they even considered Gender Flipping "surplus" males.
The titular Chakats having a certain saying, "love doesn't divide, it multiplies."
In addition, Foxtaurs are polygynous due to a 3:1 ratio of vixens to todds and todds are "obligated" to make themselves available for stud duty once every five years.
Caitians (aliens based on lions) are even worse, with an 8:1 ratio.
Stellar Foxtaurs, having been designed with the best traits of Chakats and Foxtaurs, tend to have similar views towards casual sex. Though mating habits vary with some breeds, Polars don't form permanent relationships while Deserts tend towards triads, and Starwalkers mate as a large group but carefully control breeding.
In the Futurama movie The Beast With A Billion Backs, Fry briefly dates a girl named Colleen who is living with her four other boyfriends. The main plot is centered around Yivo, a massive, hermaphroditous alien who starts a relationship with every organic being in the universe. At the end of the movie, both relationships end, as Colleen goes back to Yivo's universe with shkler, closing the portal between the universes behind them, and the movie ends with the Aesop that "love doesn't share itself with the world". Old Man Waterfall states that he loves his seven wives, and his husband.
The Netherland's first married trio. The guy was married to a wife and they both registered a partnership with the second woman, who is a partner, but not legally a wife.
"It’s the way we have been for nearly four years. I’m very fortunate. It takes some extraordinary men to make a situation like that work."
Misha Collins (Castiel on Supernatural) had a threesome relationship with his wife and another woman for some time, as explained in her book. She describes how a night of cosmos with a friend led to them all in bed together, which led to her later realizing that she had fallen in love with her friend, while still remaining in love with her husband, and that both of them loved each other and her. It all worked out in the end.
William Moulton Marston (creator of the lie detector and, as "Charles Moulton", Wonder Woman) lived this with his wife Elizabeth and Olive Byrne...the relationship was so stable that the two women continued it after his death and until Olive's about forty years later.
In September 2012, three people were married to each other in Brazil.
As stated elsewhere on the wiki, it has been reported by the news-magazine The Independent, "polygamy is a right enshrined in South Africa's constitution." South African leader Jacob Zuma is a prominent example. In 2010, he married his fifth wife in a traditional ceremony at his remote homestead. In terms of history, the news-magazine also stated, "In 1998, the University of Wisconsin surveyed more than a thousand societies. Of these just 186 were monogamous. Some 453 had occasional polygyny and in 588 more it was quite common. Just four featured polyandry. Some anthropologists believe that polygamy has been the norm through human history. In 2003, New Scientist magazine suggested that, until 10,000 years ago, most children had been sired by comparatively few men. Variations in DNA, it said, showed that the distribution of X chromosomes suggested that a few men seem to have had greater input into the gene pool than the rest. By contrast most women seemed to get to pass on their genes. Humans, like their primate forefathers, it said, were at least "mildly polygynous"."
The HBO documentary Cat Dancers examines the tragic lives of Ron Holiday, Joy Holiday and Chuck Lizza, a trio of big-cat performers who preceded Siegfried and Roy and were a romantic threesome. Their partnership ended after a tiger mauled and killed both Holiday and Lizza.