On first blush, it may sound exciting, but over time and as you mature in your sexuality the concept becomes more of a fantasy. Years ago, I came close, but no cigar; as they say! My truthful answer is NO!
I don't think I could make it work. When my fiance are having sex our attention is always completely on the other person. Just about everything else is tuned out. However, if my fiance was with another girl and I was watching and then took the other girls place, I would be fine with that.
My man has had a threesome in the past and said it was dreadfully boring and totally not hot. He certainly doesn't have a fantasy to have another one, especially since now that he's with me, he does NOT want to share me with anyone!