To be totally honest, you won't have much time for another partner I think. This next guy has to realize that he will get a small piece of time with you and that is it. Your primary I assume you wish to come first and then your secondary and then this other guy... it doesn't have to be like that, if all agree and are fine with the arrangement. You can make your time equal with all of them, but I would suspect that your primary would object.
All worth talking about and being honest about. There is no reason that dating this third guy can't be an option, but it's best to be honest that it may not be what he is used to in the past, which could be total attention. As long as he knows that and is okay with that, then you're good to go.
What does your primary think of all this? Taking on new lovers seems all fine and dandy when in the beginning stages and there is little to no investment, but as time goes on and more investment occurs it can mean a disconnect from your primary. Is it worth discovering with this guy? Besides that his best friend is your boyfriend. Is it worth it to put pressure on that? These questions are for you to answer of course, perhaps the level of depth you want to have with each of them is minimal and it's all good, perhaps you want lots of connection and depth and you will be spreading yourself out too thin? All for you to answer as you know yourself and your loves best.
In the mean time, there is no rush.... you don't have to do all of this at once. Giving it all some time and seeing if this new guy really is of interest or if it's a one off might be an idea. It's all worth talking about however as the advent of new partners could come back again in the future
Anyone want to be friends on Facebook?
Send me your name via PM