Don't know until you try? (First timer advice)

Hi!

Context:

My boyfriend (M36) and I (F28) have been together for 6 years. we just recently started playing around with the idea of MFF 3somes, found a 3rd (one of our mutual friends), talked about boundaries/likes/dislikes and set a date. This will be my first threesome and even first time with a woman (I'm bi/curious and have always wanted to explore).

I'm struggling with how to set boundaries because I feel a weird duality in my knowledge of what I'm cool with vs. not cool with in the bedroom, being that I've never tried? for example, watching my partner penetrate another woman could be something I really enjoy, or really don't enjoy. this particular woman I wouldn't mind watching him fuck, but I know on the other hand if it was a different woman I wouldnt feel as comfortable with it. is that just part of the play? walking the line between comfort and discomfort?

Does anyone have any advice for first timers with boundaries? is it better to start with boundaries then voice if I want to push those boundaries during the act, or be open with less boundaries and communicative if things get weird?

any other advice is welcome!