My boyfriend wants to try a threesome...what do I do?

So lately my boyfriend has been bringing up having a threesome with another girl. He wants to try it but I don’t know how I feel about it. I never messed around with a girl before so I don’t really know like what to do or how to even approach someone and ask them to join us!?!? I don’t know how I will react seeing him with another girl either. How do you approach a random person to ask them that? He thinks we could just go to a bar and pick up a girl but I feel weird doing that. Anyone have any advice??