From my experience: MMF (implies there will be m/m sexual contact) while MFM (implies there is no m/m contact).
I have two throuple fics, one with m/m interaction(labeled MMF) and one without (labeled MFM) so I’m think it would hold the same connotation for your category: MFM (if there is no female/female sexual contact and they’re merely sharing the male) or MFF (if the two women will be sexually interacting). Go with whatever you feel is appropriate of course, this is just my anecdotal opinion, but that’s how I’ve always read the order and I don’t think I’ve ever read a threesome that didn’t follow that pattern. Never read a MFM that had M/M and vice versa.