Threesomes

I'm really interested in hearing stories from anyone who has had a threesome. My boyfriend has a married friend he's had one with in the past & who is interested in doing one with us. The guys wife is okay with it & I'm really interested, but I've never had one before. We're meeting up beginning of February so I can meet him in person. Anyways, those of you that have had a threesome before: what was it like? Did you talk about everything beforehand? What advice would you give?