Different adaptations of the Superman mythos (i.e. the 90s' Lois & Clark TV series) will throw in Lex Luthor as a competitor for Lois Lane' affections. Results may vary.
Superman's ex-girlfriend Lana Lang is also in competition with Lois for his affections. In some versions of the story, Lana is just as unaware as Lois that Clark is Superman. In other versions, Lana has known since high school, and it was the initial reason they broke up—resulting in her rocky marriage to Clark's friend Pete Ross, making this a Love Dodecahedron. Superman himself long carried a torch for his college girlfriend Lori Lemaris, and was bitterly disappointed when she rejected him for Ronal, whom she married (Lori and Ronal are both merpeople, incidentally). Superman and Lori are now "Just Friends.".
The main trio of the Justice League, Superman, Batman and Wonder Woman actually sometimes have of love triangle sorts. Diana has been with both men on more than one occasion but Bruce and Clark would never◊ actually fight over her. There's also some unsaid sexual tension between Supes and Batman themselves.
Happens in the same fashion to Hal Jordan. At about the same time that Hal received his ring, Carol Ferris broke up with him in order to focus on running her father's business. That didn't stop her from chasing after Green Lantern whenever she got the opportunity. Carol/Hal/GL also threw Carol's alter ego Star Sapphire into the mix for extra bonus fun. Retcons have made Star Sapphire not so much evil per se, just ... persistent and perhaps a bit delusional.
Legends of the Dead Earth: In Superman: The Man of Steel Annual #5, Kaleb falls in love with Lang almost as soon as he lays eyes on her. Corin, another resistance fighter, has been in love with Lang for years and takes an instant dislike to Kaleb. It grows even stronger when it becomes apparent that Lang reciprocates Kaleb's feelings.
For a time in the Silver Age Renno the Merman, who had taken Diana on a few dates when they were teenagers before she left Paradise Island, tried to vie for the adult Diana's affection but by this point Diana was in love with Steve Trevor. Keith Griggs spends most of his time preoccupied with both of them. He flirts openly with Diana, but pays more attention to Steve.
Senator Brad Covington has long been a married man, and for about just as long as had an affair going with Lisa Abernathy with whom he has a daughter.
Wonder Girl's relationship with Ronno/Renno the merman and "Wingo" the male harpy starts out as a classic Betty and Veronica situation with W.G. going out on dates with both of them and the two boys vying for her attention but as she gets older the boys end up each other's closest friends.
During the Cap's Kooky Quartet period, Hawkeye was in love with the Scarlet Witch, but she only had eyes for Captain America (who was completely oblivious). Meanwhile, Quicksilver witnessed all of this and thought he was the Only Sane Man.
There's also the Crystal/Black Knight/Sersi love triangle, which threatened to turn into a Love Dodecahedron because of Crystal's husband Quicksilver.
Deadpool: Deadpool and Thanos are involved in an exceptionally Black Comedy version of this with the Anthropomorphic Personification of Death. They're both in love with her, but she remains willfully aloof while switching her affections back and forth from one to the other.
Fantastic Four: Reed and Sue are married with children, but Sue and Namor have had feelings for one another for years. They've only ever acted on them in alternate universes, but they've had a lot of alternate universes together. Later writers revealed that Reed was secretly also in the same situation with his first love/college sweetheart/ex-girlfriend Alyssa Moy.
The Incredible Hulk: Betty Ross with Bruce Banner and Glenn Talbot, then with Bruce and disposable love interest Ramon. Throw her and Bruce's alternate personalities into the mix and things get even more complicated.
Peter Parker has one early with Liz Allen and Flash Thompson, with Liz beginning to have feelings for Peter much to Flash's jealousy. But seeing them both fight, soon led to Liz giving up on both of them.
He also faced this problem back when he was with Gwen Stacy and had some feelings (mostly shallow ones) for Mary Jane Watson. Gwen seemed to have won, though her death sadly came soon after.
Betty Brant was the center of the triangle with Peter and Ned Leeds and the most famous with Gwen Stacy and Mary Jane until Gwen's role (after her death) was taken by Black Cat and subsequently Peter married MJ.
Ultimate X-Men (2001): The Cyclops-Jean-Wolverine triangle is here as usual (ending in Ultimatum with the death of both males), but also Rogue-Iceman-Kitty Pryde (Gambit is The Lost Lenore for Rogue in this universe).
Ultimate Spider-Man: Peter and Mary Jane are the one true couple, but they have had some crises. One was when MJ suspected that Peter liked Gwen Stacy (he didn't). At another point, he was dating Kitty Pryde. And in another, she actually dated Gwen.
The Ultimates: Initially, Pym and Jan were a couple, but things got ruined hard when he tried to kill her during a case of domestic violence. She started dating Captain America, but his views were incredibly dated, and eventually returned to Pym.
Ultimate Fantastic Four: Downplayed. Sue rejected Reed's proposal and eventually started dating Ben. Reed, however, never cared a single bit: by the time he had become The Maker and had higher things in mind than romance.
The archetypal love triangle had Jean Grey torn between stoicCyclops and bad boyWolverine. In the Krakoan Age, they all got together in polyamory: they all live together in the Summers' home on the moon, raising Jean and Scott's children. At first, it seemed like only Jean slept with each of the men (and they remained platonic friends who shared her); however, it has since been revealed that Scott and Logan are likewise romantically and sexually involved with each other.
And then there's Jean/Scott/Emma...even though "Jean's dead again", that doesn't stop the writers.
Time-travelling teenage versions of Scott and Jean got into their own short-lived triangle with X-23, Wolverine’s Opposite-Sex Clone.
There is also Gambit/Rogue/Magneto as well. Luckily Rogue went back to Gambit after Savage Land.
Young Avengers: Kate Bishop has a Betty and Veronica situation with Patriot (Betty) and Speed (Veronica), and Stature is in a tweak of the Superman variation with Iron Lad and the Vision, whose brain is patterned off Iron Lad's memory engrams. The only stable couple? Wiccan and Hulkling.
Other Comics
Archie Comics has a Love Triangle subtrope all to itself. Archie is perpetually stuck between Betty, a girl-next-door type, and Veronica, the Alpha Bitch daughter of a wealthy industrialist; the two women are frenemies. There are also the odd Romantic False Leads for all three of them, and Reggie has an unrequited crush on Veronica.
Blue Is the Warmest Color: Sabine-Emma-Clémentine. Eventually it resolves into just Emma and Clémentine.
Circles sometimes hints at one among Marty, Taye and Ken, but never acted on it. Also, Paulie, John and Arthur were an actual One True Threesome during the 1970s, and Paulie and Arthur are still best friends.
ElfQuest has a few, complicated by Recognition. This is a "mating urge" bringing together a pair that are a good genetic match to produce gifted children, and traditionally a Recognized couple will become lifemates.
Leetah's choice between Rayek and Cutter.
Dewshine, Scouter, and Tyldak (when Dewshine and Tyldak recognised, but couldn't stand each other)
Dewshine, Scouter, and Tyleet, when Scouter and Tyleet recognized, ending in a stable threesome
Mickey Mouse Comic Universe: Does Clarabelle Cow go out with Goofy or Horace Horsecollar? Some of the comics involving them play with this trope.
German artist Walter Moers once wrote such a story, involving a (male) fir tree, a (female) triangle (as in the instrument; forms a pair with the tree) and a toblerone bar (male, bisexual) who cheats with the triangle. While they're in a tent. (Triangle / triangular story, get it?) As the author Lamp Shades: Doesn't that sound pretty constructed?
Subverted in the erotic furry comic, Omaha the Cat Dancer. In this series, Omaha runs away from her lover, Charles Tabey Jr., in utter desolation after he blew up on her for a dark secret she had to confess (She was already married, had broken up with her husband long ago, but never actually divorced yet) and goes to a small town where she starts a passionate relationship with another man. However, Omaha learns that Charles was hospitalized with a nervous breakdown because her leaving him and she decides to return to him. Before she does that, she explains the full situation with her new boyfriend and he is understanding enough to encourage her to go back. After all differences are patched up, Charles makes a point of inviting that man to their New Year Eve's party to show his appreciation for helping Omaha at that tough time he himself put her through. As it happens, the pair get along so well they agree to take Omaha to bed a threesome and a fun time is had by all.
Defied in Pathfinder. The first issue has some light flirting between Valeros and Merisiel, the latter of whom even says that she isn't in the mood for him at the time but might be at a later date. She starts a relationship with Kyra in issue 6 (they eventually marry in a Short Story on Paizo's blog), and Valeros accepts this without complaint when he finds out.
In Pocket God, Nooby has a crush on Sun, but get jealous when she starts hanging out with Ooga. Nooby gets over it by issue 21, when he realizes they still care for him.
Quantum and Woody has this with Eric/Quantum carrying a torch for Amy Fishbein, who's got the hots for Woody (C). Woody is indifferent to Amy, but knows that Eric's infatuation with her has messed him up a bit.
Woody: "The only reason I'm even wearing this stupid costume is because you couldn't score with Amy Fishbein!"
Sin City is normally not the sort of place you might expect a love triangle, but we have at least two:
Dwight bounces back and forth between Gail, the badass prostitute and Shellie, the sweet waitress.
Then we have an All Love Is Unrequited situation. Marv loves Nancy, who loves Hartigan, who only loves her in a platonic manner. It's a rare example in that Marv and Hartigan never knew each other.
In SpyBoy, Alex has a liking to both Bombshell and Spy Girl.
Strangers in Paradise, most of the time. David longs for Katchoo, Katchoo for Francine, and Francine for David.
Star Trek: Early Voyages: In "Thanatos", José Tyler is extremely jealous when he sees the Enterprise's new assistant chief engineer Shane Samson flirting with Mia Colt, with whom he is madly in love, in the mess hall. The two men almost come to blows but they are interrupted by a red alert.
In The Walking Dead, Carol grows feelings for married couple Rick and Lori Grimes, constantly telling them both she is in love with them, and suggesting to make a polygamous relationship work and raise one another's children together. However, she is shot down by both Rick and Lori on this idea, and berated for not focusing on being a mother to her daughter Sophia.
◊ actually fight over her. There's also some unsaid sexual tension between Supes and Batman themselves.
