I have never, EVER heard of a good outcome from a threesome. No matter how close the couple was, no matter how much they discussed/planned it ahead of time, it didn't end up as they had imagined it. Just like that first responder mentioned, jealousy does come into play and YES, more often than not one partner finds themselves wanting more with the "other" person after the threesome is over.

I'm not saying don't do it, because it's your decision and I can't make it for you. But I'm letting you know that I personally know many people who have been involved in threesomes, and there has never been anything good to come of any of them. So really think about whether it would be worth it once it's all said and done.