Well this all depends on whether you have interest in a threesome or not. If you do then explain to him that you are willing to do that, and talk to your friend. Tell her that you and your boyfriend would like a threesome is she interested. If you are not slightly interested then you need to be straight and tell your boyfriend that you want nothing to do with a threesome with your friend or anyone else. If he's a good boyfriend then he'll drop the idea immediately.
If you are not sure about it then i would put it off as well. Only do it if you are 100% certain that it's what you want. Don't let your boyfriend pressure you into doing things sexually, because you do not need to. If he persists with it then you need to rethink your relationship. Any guy who puts his sexual needs above his girlfriend is not a guy worthing keeping.
I wish you the best of luck and hope that all goes well.
-Cara