Tsukushi falls in love with Rui, while Tsukasa falls in love with Tsukushi. Rui loves Shizuka but this is an unrequited love. When he gets back he slowly falls for Tsukushi who falls in love with Tsukasa instead. A love triangle from the Hana Yori Dango franchise.
Soshi, Riiko & Night
A love triangle from the Absolute Boyfriend franchise. Riiko's neighbour and childhood friend Soshi is secretly in love with her, but Riiko orders a boyfriend doll of the internet who is literally made for her. She now has to choose between Soshi and her robotic lover Night.
An early example of an anime love triangle is the Allura-Keith-Lance one that was frequently brought up but underdeveloped in these series. Apparently Allura chose Keith because the ghost of her father said he approved of him...