I've had only one MMF threesome. One guy was my best friend, the other a fuck buddy. One bisexual, the other bi-curious.
Initially, there was little interest on my part; I did it to facilitate my bi-curious friend's first MM encounter. Once we started, though, I loved it. Partly because I love attention, partly because facilitating others' fantasies is such a turn-on, and partly because it's just so much FUN! One of them was quite a bit older and MUCH more experienced, as well as trustworthy, so we knew we were in good hands.
On the trust factor, I must say it made all the difference for me - I have no romantic feelings for either of them; instead I would describe my feelings as deep trust for one of them, and deep platonic love for the other. Those feelings were intensified during and after that encounter.
And now I get to say I've been spit-roasted... <3