I have tried MMF and love it. Adding one extra person sometimes can more than double the fun. However, the key word is sometimes.

MFF, never tried, but would not mind to try.

FFF..... NO way I can't just watch without participating lol

MMM...... errr.... I don't think I can do that.

I agree with that the one on one is still the best. Never have to worry about neglecting anybody, and you can concentrate 100% on your partner.