If you don't feel comfortable with the threesome or are unsure about it then let your partner know that, don't do anything that you are uncomfortable with just to please your mate, remember you have to be pleased also. The thing about threesome is it take a certain amount of trust in not only your partner but the third party and unless you have someone that you both know in mind it could be a frightening experience. Before you decide to invite a third person into your bedroom, you have to set rules regarding what is forbidden, when you and your female partner include another female you felt more comfortable accepting that, if you and your female partner include male, then you may to want her not want her to be penetrated the male sex partner. This is a very touchy issue, and if the rules are broken, the whole experience could come to a grinding halt or worse, you may wind up losing some trust for your girlfriend and never have another threesome. If you do decide to have a threesome with your girlfriend and another man; you would probably want to set some ground rules regarding his behavior.
You and your partner must remember at all times that you are having an experience with a third party, not a relationship! At moments it will feel impossible to keep a cool distance, but if you cannot keep the third person at an emotional distance then you run the risk of damaging your relationship. Intimate experiences cause emotional connections to form. You may have to disconnect if you decide to do the threesome so that the emotional bonds does not occur. You have to know when you are ready and if you don't think you will ever be ready then communicate that to your partner as soon as possible. You have to have a certain about of comfort about the threesome that you can disassociate yourself from the situation so not to feel uncomfortable or your partner should allow you to make the final decision about who the third person is but you have to really talk to your partner and tell her what you want and don't want.