threesome?? help!

so basically I've never had a threesome before and this guy I casually hook up with asked me if I wanted to with another girl. like I'm into that but not with another girl. I would be 100% on board with 2 guys and me but idk how I feel about another girl. have any of you had a threesome? I think I'm mostly just intimidated by me possibly being bigger and not having much attention.