Threesome advice?

Lately my boyfriend says he's been fantasizing about me with another girl and wants to try a threesome. I have always been curious to try a threesome myself, but I'm worried I'm going to get jealous and it will complicate my relationship. He says he's fine with just watching and doesn't have to do anything with the girl. Has anyone been in a FFM threesome, or a similar situation? How did it go for you? Should the third person be a friend or a stranger? Anything else I should know?