There's a strong argument to be made that the entire sub-genre of Pop Punk is built on this trope.
The They Might Be Giants song "Withered Hope" is about about a Love Pentagon that's like this. A Very Sad Sack loves Withered Hope, who has a soul mate, who loves a picture of a crook, who loves a motorcycle, who loves Very Sad Sack…
The Burning Hell's song "Grave Situation, Part 3" describes a love quadrangle involving "a man who loved a man who loved a woman who loved another man"
In "Living Next Door to Alice", most successfully covered by Smokie, the narrator has been in love with his neighbor for 24 years, but couldn't tell her. And now she's moving away. Meanwhile, the narrator's friend, Sally has been in love with him for 24 years, but he ignored her because of Alice.