Here are some others.
A Midsummer Night's Dream - Everyone: This one is rife with twisted relationships, thanks to Puck and his magical goof. Who doesn't love watching Titania fawn over donkey-headed Nick Bottom?
The Great Gatsby - Gatsby/Daisy/Tom: Tom and Daisy are married, Daisy and Gatsby are old flames, and Tom's not too fond of Gatsby for obvious reasons. This one was doomed from the start.
Les Misérables - Cosette/Marius/Éponine: Pre "Team Jacob/Team Edward", there was "Team Cosette/Team Éponine." This one is a bit more dramatic than the vampire/werewolf feud.
What other classic non YA love triangles can you name?