First time Three some

So my boyfriend and I were talking about fantasies and he told me he wants a three some with me , him and another girl.This will be a first for all 3 of us. I don't know how I feel about it yet or what my decision is. I've never even imagined the thought of messing around with another girl or sharing him with another girl. Has anyone else been in a three some ? How did it go ?What are some things that should be talked about before hand between the 3 of us? I'd love to hear other peoples experiences and any advice is appreciated