Boyfriend wants a threesome, if I say yes (I'm still thinking if i want to), What should we do to prepare if it does happen?

I just want to know some basic guidelines I guess. Some do's or don't's for this. I'm on the fence on it but I'd rather know some more info on what would be best to do between me him and the girl he wants us to have a threesome with, in case i do go through with it. If it happens, It's not gonna happen any time soon since me and him work so much. I'm not on birth control and i am in the process of getting myself under a women's health Care program so i can get birthcontrol. I know i want to wait till i get that done first. I know I may want to have us all do an STD test before it happens Not because i don't trust him. Even if he trusts her, i don't know her and I think it would be best for everyone's health safety too. Other than those things I'm not sure what else to do? That's if i go through with this.