well it depends. like in inuyasha there was a major triangle. kikyo and kagome. that really annoyed me tho was the fact for the manga every three books there would be a inuyasha,kagome kikyo thing. then it really got on my nerves seeing inuyasha go back and forth from kikyo to kagome. like i said it depends. but when it goes to a certain level,thats it.
My favorite is Kyo/Tohru/Yuki in Fruits Basket *insert total non-shockedness here*, but I also like Zero/Yuuki/Kaname (Vampire Knight), Tamaki/Haruhi/Hikaru (Ouran), and Mizuki/Akihisa/Minami (Baka and Test) as well as some others that I'm pretty sure I made up.
I don't, because in triangles, there is usually a couple that I love, but the other one disgust me (*thinks about Vampire Knight at the moment)... So, when there are moments between the couple I don't like (well, it's ok if I don't like the couple, but if I can't stand it...), I have hard time watching it... Sometimes, I even dislike a character just because I don't like him/here paired with someone... I know it's stupid and I doesn't happen often, but I those times, I can't help it XP