Some books that could partially fit:

• In Madame Bovary by Gustave Flaubert a boy loves a woman who is already married
• In Anna Karenina by Levy Tolstoy a married woman falls in love with a young man
• In The Scarlet Letter by Nathaniel Hawthorne a man loves a married woman
• In The History of Tom Jones, a Foundling, by Henry Fielding, a poor boy likes a rich girl, but the father of the girl does not agree, but they end up together
• In The Betrothed by Alessandro Manzoni, the boy wants to marry the girl, but a very rich and powerful man is in love with the girl and want to marry her too
• In The End of the Affair by Graham Green, both the man and the woman are with someone else and they end up together
• In Twilight by Stephenie Meyer there is a teenage love triangle with warewolves and vampires