IMHO, that is. It depends on how well versed everyone is and whether they care. (I know you local sluts want sex probably know the rest below, but I'll throw it in here for the lay reader) Traditionally, the convention is that the letter (M or F) for the central partner, the one who's expected to be receiving the majority of the attention from the other two, is placed in the middle position. Therefore, a MMF would differ fundamentally from a MFM. In the former, you'd have a and a woman tag teaming a third male and in the latter you'd have two men ganging up on one woman. So if you're a couple and you want to invite a to play with but don't want any 'sword crossing', you'd best be inviting him to a MFM and not a MMF.