My (21F) best friend (20F) offered a threesome with herself and her 20M) boyfriend, what boundaries are good to set?

Hello all, I needed some advice on a situation that I've recently found myself in.

My best friend, K, soft-launched the idea of a 3some with me, her, and her boyfriend, A, a few days ago and then brought it up again last night.

I am very much not opposed to the idea. I have previously joked about being their third and am currently single and very pent up. They're both very attractive and freaky in the sheets, so it was never out of the realm of possibilities. I just never actually expected to be propositioned, lol.

She and her boyfriend are very enthusiastic about the idea and told me to think about it and what I'd want boundaries to look like for myself, and I'm not too sure what things are normal with something like this?? like are there blanket boundaries that can be used for any threesome? Do I give a list of things I’m into sexually? I’m not 100% sure how to proceed. I don't think this would damage my friendship with this person at all, and it would be a win for everyone involved. I’m just a bit frazzled and have nobody else to ask about it.

Any and all advice is appreciated!! Thank you!