Ill begin with: PEACH GIRL!

Such an awesome anime!

that's tough, most my favorites are much more complicated than triangles, i'll have to think some more,

Nyarko, Kuuko, and Mashiro in Nyarko-san: Another Crawling Chaos make a fun comedy trio. Nyarko (silver hair) usually makes some ridiculous advance on Mashiro that annoys him. Then Kuuko (red hair) makes the same kind of advance on Nyarko, which ends up annoying her just as much. Then there's Hasta, making it more complicated than a triangle, but he's often not in those scenes.

Fruit Basket

Vampire knight

Gakuen Alice

Inuyasha