I've never had a poor FFM. But there are things that detract rather than enhance the experience for me - if one is sat on my face for example. Happy to lick one girl's holes while the other attends to me but having an arse covering me I guess makes it feel like it could just be one of them with me.
Some things are unbelievably good in a threesome though - DFK while the other sucks my cock, three way FK, rubbing my cock between their pressed together faces, tits or arses, fucking one doggy while the other one kisses me and strokes my balls and bum crack, having one slowly wank me while I watch the other undress, bend over, stretch their holes etc - it's all good!!!! And 4 hand massages are fucking awesome!!!!
But the key for me is to be in control of what they are doing otherwise I can easily see how you'd not get what you want from the session and go away disappointed