A threesome with two males and one female. This variant of group sex is nowhere near as popular as the MFF, which virtually every red-blooded young heterosexual male fantasizes about at some point, but it does show up from time to time. MMFs come in two flavors, one relatively common and one extremely rare:
1. Heterosexual MMF. Both men have sex with the woman, enjoying double penetrations, oral tradeoffs, oral/anal and oral/vaginal combos, but they never, ever, EVER so much as touch each other (because THAT would be ICKY!). This type of MMF shows up in porno mags quite regularly, no doubt because it's kinky but relatively cheap (since male models cost much less than females do).
2. Pansexual MMF. Everybody does everybody, in as many ways and as many positions as creativity and flexibility allow. Typical poses include the double blowjob, in which the woman and one guy fellate the other guy; the reverse cowgirl oral, in which the woman and one guy perform the reverse cowgirl while the other guy laps away at both of their naughty bits; and the double fuck (or triple fuck, depending on your perspective), in which the Lucky Woman is penetrated by Guy 1, who is in turn penetrated by Guy 2. This sort of MMF is extraordinarily rare in porn and presumably in real life (I wouldn't know about that), because it requires two bisexual males and a woman who doesn't mind watching the guys go at it or boffing a guy who's just boffed another guy. If you're looking to fill an unfilled niche (heh...sorry) in the porn business, this is what you'll want to produce.