The threesomes will be FFM, and will involve only the mc. I think I have to say swinging will technically be a part of the game, but it will only be the mc who will be having sex with the women, not their current partners. In other words, the women will be shared with the mc; he will not share them with others. The reasons why the women's current partners aren't having and can't have sex with the women hasn't been made totally clear in the story yet, though, since I'm trying to create a sense of mystery around what's going on with them (the women).
I hope that makes sense. It's a bit difficult to talk about without spoiling some things. I'll give some thought about a list of fetishes that seem like they could be included but won't. MMF threesomes would definitely be one of them.