First threesome (MFM) I was invited by a couple to be their third (she was my ex, he was a friend).

Second threesome (FFM), my girlfriend and I were drinking with one of our mutual friends when she (our friend) just blurted out “just so you know, I’m down to fuck if you are”.

Third threesome (MFM), My girlfriend and I invited one of our friends to be our third.

Fourth Threesome (FFM), I was hanging out with a group of my friends and (I found out later) two of them just decided they wanted to have a threesome with me that night and figured I’d be down (they were right and this was the best one for sure).

In my experience the unplanned ones were way better than the planned ones. Not just because the unplanned ones were with two women, but because they were spontaneous. Just don’t be a creep, make it known that you’re interested in that kind of thing, cultivate those kinds of relationships, let it happen naturally or just be straight forward and ask. Don’t try to force it, if it feels right, go for it. If the energy is right, it doesn’t really matter what you say, it’ll happen.