The trick to getting your girlfriend to having a threesome is to make her think it is her idea. This sounds difficult, but it really isn’t you just have to know how to talk to her about it. You shouldn’t tell her what you want out of the threesome, but make it all about her. She needs to think that a threesome is the answer to fulfilling her own desires and not yours. If she mentions a threesome in passing, or when you ask what her fantasies are, then jump in and ask her how she pictures it.
You can tell her you want to make that happen for her, and that you want it to be a great experience for her. Tell her you will only sleep with her and tell her what you and the other woman will do to you. Don’t mention wanting to bang the other woman or you will be shut down quickly. Don’t be over eager about the other woman; remember you are excited about experiencing it with her. You will get your time with the other girl, but you cannot let her know that this is what you are thinking about, or it just won’t happen.
You can never reassure her enough that it is all about her, even if you really want them all over you. The only way you are going to make this happen is if you continue to use language that appeals to her. Tell her how erotic it will be for you to watch her with another woman, and how hot she is. Never say how hot the other girl is, or if you do reassure her she is hotter. So good luck, don’t get too excited, don’t ignore her during or before the threesome and everything should go great.