My personal parameters are: do what feels good given the chemistry and the players.

I've done more MMFs than MFFs, and there was MM contact - some incidental, and some on purpose. It's all good.

I wouldn't have a 3sum of either kind with someone whom you wouldn't be interested in alone. If you wouldn't be into them in a 2-way, then you're not going to be down with a third person there too. Now, sometimes personality is more important than looks regarding their attractiveness - I've slept with women I thought were cool but weren't really my type. That's not what I mean here.

Weight is the same as the above - go there if you're into the person, but don't if you're not.

Basically, if you're doing it just to tick a 3sum off your bucket list, then that person (or the other couple) is unlikely to enjoy being with you, and you will pick up on that unless you're a totally delusional asshole :)

The woman involved in my threesome was overweight and not really my type. Not ugly, just plain. But I took the opportunity anyway, because why not? She pushed for the semi threesome nature and I didn't argue. I don't have an interest in dating her, but I'd fool around again. Sometimes sex is just sex.

am i mixing my terminology here? is there a difference in an mmf or mfm or fmm?

No - there is no broadly accepted difference between those acronyms. Some may think "the one in the middle is the one that gets the attention" but that's not always the way they work.

I was confused by the statement too.

In some circles - in some contexts - mmf is used instead of mfm to indicate that the m's involved are not opposed to male-male contact. In those same circles/contexts, mfm is used to clearly indicate that the m's involved are strictly there for the f.

So - it depends on the group.

It's interesting, the more I fantasize about this the more possibilities run through my head.

How would I react to being in bed with two women...to a certain extent the most exciting part would be to watch the two women with each other. The outside observer into something amazing. But also the idea of having both of them on their backs enjoying my fingers and tongue.

What about two men and a woman? The pleasure of worshiping a woman so completely. Then there is also the side of me that is dying to watch and participate as the third person with a couple. I see myself holding and fondling the two of them as she rides him reverse cowgirl style. In that position I can see and feel her lips expanding to accept him, bring added pressure to her clit as well as use her wetness to stroke and run my thumb against his super sensitive shaft. The most exciting thought there is to be holding him as he is cumming inside her. Feeling his muscles spasm and fill her...

whoa, gotta stop