3 some??

Im bisexual but Ive never been with a girl before, I want to try it but I’m currently with my boyfriend and I’m not about to cheat on him with some random girl so I want to ask him about having a 3some. the only problem is, is that I’m worried about how I’ll feel afterwards or during cause I want him all to myself but I want to experience sex with a girl, what should I do?