Threesome

I just started dating my boyfriend and he says he loves the fact that I'm so open to all things, (not just sex wise) but he was talking about a threesome, I have slept with a good amount of people but never have done anything with a girl or this at all.... I don't know how I feel about it but it makes me nervous I don't even know how I would do that or any girls even down to do that... What do I do...