Cilea's opera Adriana Lecouvreur has a love triangle between the French actress Adriana Lecouvreur and the Princess de Bouillon for Maurizio, the Count of Saxony.
Meyerbeer's L Africaine has this between African slave girl Sélika and Admiral Don Diego's daughter Inès for naval officer Vasco de Gama.
In Verdi's Aida, Princess Amneris of Egypt loves the warrior Radamès, but he's in love with her Ethiopian slave Aïda.
The Aida musical adaptation ultimately contains the same triangle, but unlike the opera, it doesn't fully form until near the end of the first act (the opera begins with Radames and Aida already a couple).
Richard Strauss's opera Arabella has Zedenka pursue Matteo who pursues Zedenka's sister Arabella—complicated by (and eventually resolved thanks to) rich suitor Mandryka who wants Arabella.
Rossini's The Barber of Seville has Count Almaviva and Doctor Bartolo competing for the affections of Rosina, who loves Almaviva. This is one of the few love triangles with a happy ending for all.
The Czech opera The Bartered Bride by Smetana has a love triangle between Jenik and Vasek for village girl Marenka.
Bizet's Carmen has two love triangles: one with sweet village girl Micaela against Femme Fatale Carmen for soldier Don José; the second one being Don José against bad boy toreador Escamillo for Carmen. The latter is perhaps the deadliest love triangle in the entire opera.
Cavalliera Rusticana, dear God. Turiddu, a soldier, has an affair with Santuzza, a village girl, to get revenge on a married woman named Lola; however, Santuzza loves Turiddu. Lola is driven with jealousy so she and Turiddu both mock Santuzza and scorn her, and Santuzza ends up telling Lola's husband Alfio of the affair, who then challenges Turiddu to a duel, in which he is killed.
La Donna Del Lago by Rossini has a love triangle between rebel chieftain Malcom Groeme and Uberto di Snowdon for Elena, the Lady of the Lake.
Elisabeth: Elisabeth is the center of a love triangle with her husband the Emperor Franz Joseph and with Death himself, though her husband does not even know that der Tod exists. Death, however, is fully aware of the situation: He tells her at her wedding that she will regret the choice she has made because she loves him and not Franz Joseph, and he is right, despite Joseph's unconditional love for Elisabeth no matter how far she pulls away.
It is an old story, but for me, new Two have the same beloved — namely, you.
Donizetti's L Elisir D Amore has one between simple peasant Nemorino and sergeant Belcore for the wealthy Adina. However, unlike most of these examples, it ends well.
La Favorita by Donizetti has Leonora di Guzman caught in a love triangle between Alfonso XI, King of Castile, and her lover Fernando.
Finale has Alex, who is in love with Tyler, who is in a relationship with Dylan, but is really in love with Alex.
Ponchielli's La Gioconda actually subverts this. At first, Gioconda seems to be competing against Genoese lady Laura for the affections of Prince Enzo Grimaldo. However, Gioconda's love for her mother prevails, as after finding out that Laura saves her mother's life, Gioconda puts aside her own feelings for him to repay Laura and helps both Laura and Enzo escape before sacrificing herself.
The History Boys: Dakin and Irwin are attracted to each other, though circumstance keeps them apart. Posner loves Dakin, but Dakin does not return his feelings. Slips into Love Dodecahedron, as Dakin also has a purely sexual relationship with Fiona, and some actors play Scripps as interested in Posner.
The love triangle in Janacek's Jenufa is between Laca and Steva for the eponymous heroine.
The relationship between Eliza, Professor Higgins and Freddy in My Fair Lady is considered this.
Yerma, Juan and Victor in Yerma, though Yerma and Victor’s relationship are more like the romance that could, and perhaps should, have been.
Les Misérables has a version of this with Eponine, Cosette, and Marius. Eponine has intense feelings for Marius, who is blissfully unaware. When he and Cosette meet, he asks Eponine to find her for him (unaware of the pain he's asking her to take for him), and she agrees, leading Cosette and Marius to fall in love and eventually marry. Eponine is eventually shot while delivering a letter from Marius at the barricades to Cosette at Rue Plumet, and dies in Marius's arms after finally confessing her love for him.
Jean-Paul Sartre's play No Exit: Garcin and Estelle want each other, but Ines, who wants Estelle, keeps preventing this.
Subverted in Bellini's Norma. Even though Norma still loves Pollione, despite him abandoning her for Adalgisa, neither Norma nor Adalgisa let it ruin their friendship and they both confront Pollione in one of the famous trios.
The Normal Heart: In the backstory, Ned unrequitedly loved Bruce, who loved Craig, who had a crush on Ned. However, the play opens with Bruce and Craig happily in love in a long-term relationship, while Ned's one-sided crush on Bruce continued.
There's Mozart's Le Nozze Di Figaro, with a love triangle between Count Almaviva and Figaro for Figaro's own fiancée Susanna.
Nadir and Zurga both share feelings for the priestess Leïla in Bizet's The Pearl Fishers. Ironic, since earlier, they sang the famous duet 'Au fond de temple saint' in which they swear that their friendship wouldn't be ruined over Leïla.
Claude Debussy's Pelleas Et Melisande, where Pelléas is up against his half-brother Golaud for Mélisande, who is married to Golaud but loves Pelléas.
In Donizetti's Roberto Devereux, Queen Elisabetta (Elizabeth) is in love with Roberto Devereux (Robert Devereux), who's in love with Sara, the Duchess of Nottingham. Naturally, this ends in tragedy.
In Strauss' Der Rosenkavalier, there is a love triangle between Marschallin Princess Marie Thérèse von Werdenberg and Sophie von Faninal for Octavian, Count Rofrano.
Dvorak's Rusalka, with Rusalka the water spirit and the Foreign Princess competing for the Prince's love.
In All's Well That Ends Well, the protagonist, Helena, a countess' ward, falls in love with a high-status man who won't give her the time of day. She secures his hand in marriage with the help of the King, but can't make him love her and he runs off to Italy where he becomes infatuated with another woman, Diana. Diana and Helena end up conspiring (with a large helping of Values Dissonance) to get Helena's husband to fall for her.
A Midsummer Night's Dream has multiple love triangles among the same group, due to Oberon and Puck's interference. Initially, Hermia and Lysander love each other, Demetrius also loves Hermianote which wouldn't be a problem for Hermia and Lysander except that Hermia's father prefers Demetrius, and he has the authority to make the choice for her and Helena loves Demetrius, who couldn't care less about her. Seeing this, Oberon orders Puck to use a love potion on Demetrius to make him fall for Helena, but Puck gets Lysander by mistake, causing Lysander to fall for Helena. They ultimately straighten it out and manage to put things as they originally intended — Hermia and Lysander back together, and Demetrius in love with Helena.
Played with in Othello. As part of a plan to drive a wedge between Othello and Cassio, Iago convinces Othello that Desdemona and Cassio are having an affair, but not only are they not actually involved in any kind of relationship, they don't even have feelings for each other.
For a good portion of The Two Gentlemen of Verona, both Proteus and Thurio are attracted to Silvia, who is in love with Valentine.
Twelfth Night has a perfect triangle version by way of cross-dressing. Viola, disguised as a boy and using the name Cesario, takes a position in the service of Duke Orsino, only to end up falling for him. Orsino, meanwhile, is pursuing a beautiful but reclusive countess named Olivia, and sends "Cesario" to deliver a profession of love by proxy, which Viola does so well that Olivia is swayed and falls in love — with "Cesario". It's Lampshaded by Viola in a famous monologue from the play.
Viola: How will this fadge? My master loves her dearly. And I, poor monster, fond as much on him. And she, mistaken, seems to dote on me.
I am a lass who alas loves a lad Who alas has a lass who loves another lad Who once I had In Canterbury.
La Sylphide has two love triangles. The first is between a young man named James, his fiancee Effie, and the titular Sylphide. The second is between James, Effie, and another young man named Gurn.
A one-sided love triangle appears in Puccini's Tosca, with good-hearted painter Mario Cavaradossi competing against corrupt Baron Scarpia for singer Floria Tosca. Though, calling it a love triangle would be a stretch, seeing as Scarpia doesn't really love Tosca and wants to have his way with her. Even Tosca hates Scarpia for endangering her lover Mario.
Tristan und Isolde has one between Tristan and King Marke of Cornwall for Irish princess Isolde.
Verdi's Il trovatore has a one-sided love triangle, with the corrupt Count di Luna against the troubadour Manrico for the affections of Leonora, who is in love with Manrico.
Yet another Rossini opera, Il Turco In Italia, has Fiorilla, the unfaithful wife of Neapolitan Don Geronio, caught in a love triangle between Prince Selim and Narciso. There's also a love triangle between Fiorilla and Turkish woman Zaida for Prince Selim. And if that weren't enough, there's another love triangle between Geronio and Prince Selim for Fiorilla.
A Very Potter Musical includes a love triangle between Ron, Hermione, and Draco. The boys realize their feelings during the Yule Ball, when Hermione arrives and everyone discovers that She Cleans Up Nicely.
In the musical Violet, the title character is in one with with Monty and Flick.
This gets a bit complicated in Wicked. Elphaba starts to develop a crush on Fiyero, who's in a relationship with G(a)linda, and the former is convinced that he would never look at her twice when he could be with the latter. But Fiyero actually does start to develop feelings for her too, and realizes in the second act that his care for Glinda is just platonic and Elphaba is the one he's actually in love with, causing the two girls to switch places.
