Anon reply - OK, you need to sit down and sort out some boundaries. Talk to them and find out what is comfortable to everyone, I am sure the bloke will not want to feel left out, or have you run off with his girl !! Decide rules like no-one left out, unless they want to watch, one hand on someone else at all times, everyone getst o come three times etc etc. Theres a lot of info on the net, but as I am at work I dont wish to be caught looking up "how to have a 3 way" info.
In my experience, these things are best if left unplanned. Once you've done it, then the opportunity is there for it to happen again but it doesnt seem to work if you say "COme over and we'll do it" Its far more fun for it to happen randomly.
How old are you?
PM me if you have any more questions.
I too am a scaredy cat so will be anon here too!
I'm no expert on this matter, but remeber to think abut your sexual health too.