Other, bisexual, female.
I've attempted to have two threesomes before with my boyfriend.
The first one was going well (despite me not being into the girl at all, he had actually slept with her prior to the start of our relationship), but it was at a college party and his door didn't have a lock so we kept getting interrupted and the girl eventually opted for another time.
The second time neither of us were really into it, which was surprising mainly because we'd both had crushes on this second girl in the past. Though that wasn't so much a threesome as much as us three making out and messing around.
My boyfriend and I concluded that we're going to stop really trying to have threesomes. If it happens spontaneously, then why not give it a try. Otherwise, we'll just stick with each other. =)