GAMRican saidMake sure you all are into each other.
One person should subtly orchestrate.
This is probably the MOST important part of a 3-some. Back when I was much younger, I had met a guy online that I really hit it off with. I was in his city and so we met up and we just totally clicked and had an awesome time. The next night he wanted to include the guy he was dating so who was I to say no? It ended up that I was NOT in any way into his BF and yet felt obligated to enjoy both. It was terrible and I didn't enjoy the evening AT ALL! I had another experience with a couple that was awesome and they were both on the same page with me. It can work, just get your ducks in a row first.