With something like this any boundary is fair as long as everyone involved is aware and agrees beforehand.

And don't accept it if anyone involved says they have no boundaries. They do but they're not comfortable saying it which only causes problems.

When I had a threesome my boundaries were clear: Finish with me. Make sure I orgasm. And anyone can stop it any time and not have to explain why to anyone.

This was with my last partner and even though we had fun and had loads of respect, I never really became completely comfortable with it. When I decided to stop it put a big rift between us that I think contributed to the end of that relationship though it wasn't the main issue in the end.

So hopefully me sharing my experience helps you feel more comfortable with your boundaries and comfort levels.