Threeways or Fourgies?

Ever since that questions was asked of Marc in Trick it has been something I jokingly bring up from time to time....but I am just curious how many people have ever, or would ever participate in a threeway or fourgy. Are there any rules you would have to abide by in order to play nicely with others?

I know if I were going to partake, I would have to be single and either the outsider in the group, or at least not overly interested in one of the guys involved, otherwise I think I would get jealous of the attention being lavished on him. I could also do it if I were the main focus and no one touched anyone but me, (think "Alex" from the movie Threesome.)