I did a 3some once. It's largely overated, imo. The events that led the girls to my bed were way more interesting than the sex in itself.
Never did with only 1 girl and another guy, though. I will be honest, I dont know if I would be able to. It probably would have to be a girl I dont care about, as it was with the 2 I had the 3some. But the other guy...I dont know.
I did participate on an orgy-like once, but it was more like people having sex all around the place than an actual orgy where people have sex with whoever. I dont mind people watching me, but I dont know if I can have sex with another ***** so close to me. I dont like to sound so sexist, but it is what it is.