As I said often here, with my gf I'm considering doing a threesome (MMF).
Actually I think the real obstacle is to find the right guy; we don't want a stranger, but not even a close friend. A good option would be her ex bf, scoring 8 inches. It wouldn't be so dangerous as it can seem, considering they were just lovers with very scarce emotions involved, but we're still unsure about it.
This said, I told my gf sometimes that I don't know what I'd end up doing in that case.
For example, I'd really like to lick her while he's inside her. This would mean, of course, almost licking him. She answered that it could be a huge turn on for her to see me sucking him, and guiding him inside her - but she's worried it could be too much for me.
She thinks about the threesome mostly because she would really like to try double penetration, and because she'd love to suck a man while being fucked by another.
In that scenario, it's difficult to understand which would be the boundaries, what could or would be done.
I think that, if a couple is open and strong enough to try a threesome - WITHOUT ANY FEAR OR SECOND THOUGHT - probably there would be no limit at all.
Maybe you can tell her the same; in a MMF, you don't know what you'd do (and NOT do). This way you may ask her what she'd like, what she'd accept, and on the contrary what she don't want to see or to do.