It depends entirely on the people concerned and the situation. Someone could feel left out in a group of any number. It is just as easy for someone to feel left out of a threesome as it would be a foursome. I can't think of any specific examples off the top of my head, but I can't honestly say that a threesome would be better than a four in orde to avoid this possibility.
Does that answer the question?
hmmmmmmmm. Personally, i think if everyone knows what everyone else wants then technically nobody should be left out. Daz and i have had a couple of very fulfilling foursomes,with a fantastic couple, at some point Daz may have been doing something with the other woman, but his hand is always somewhere on me. We have also had the other extreme, we had a foursome where the man was only interested in full sex with me,(which we werent interested in) and his wife said she was bi but actually wasnt, and was really reluctant to let Daz anywhere near her. Had the male had his way, Daz would most certainly have been left out - as it was, we made our excuses and left!
Three is cool. Four is cool. Five is... cool.
Um, I think that you need to get the right people together. Make sure you all like eachother the same, ya know? The only times anyone has been left out, in my experiences, are when I fall asleep. (No slur on anyone involved, I am ill.... it happens).
I agree with Blue and Seagll, it is to each persons attitude, imagination weather they chose to and how much to participate.
The more people in the sesh the more things that can be pasted around.
In theory, there should be more activity going on in a 4 some then a 3 sum.
Shaddup, Mr sensible. We're trying a hi-hack, here!
any thing else kicking in??
It sounds like you 2 have a conspiracy going on
We have generally met up with couples for fun but have had our more sexually intense experiences where we have met single guys for threesomes. With 4somes, there is a tendency to split into two couples with occasional interaction; with MFM, it has been continuous interaction and M has been in her element! Naughty girl!
I have a technical question about this. I have never participated in a threesome before (foursome yes)... teh nearest i got to a threesome was years ago with two men whom I had to in the end kick out my door as they both started squabbling about you gets to have first ermmm penetration..
My question is, in a MMF set up, how does the F make sure that she doesn't get absolutely knackered??
Personaly I prefer swaps with other couples who are both bi i can assure you no one gets left out then. On the other hand I have found that in a threesome if two people get carried away with each other then the other person can get left on the side for a bit but again it is down to individuals to make sure everyone is involved. as a couple we have had threesome and moresomes and we think a bloody great big bi orgie is the best 
. How about sharing? Oops, sorry, you did that before & after the wedding! 