Threesome w/ boyfriend

So my boyfriend asked me for a threesome. He told me that i can be the one to find the girl. I’ve always been curious and interested in having sex with a female so thats not the issue, the issue is idk how i would feel in that moment watching my bf doing sexual things with another female in front of me. Like what if he enjoys it so much that it makes him want to keep having sex with other females. Or what if the other female is the one to make him finish and not me? Im i overthinking too much? Lol

Have anyone had a threesome with their bf? Was is weird after? What type of rules/boundaries did ya have in place? Any advice?