Sounds like you have to convince both of them. Some honest advice... don't go there. Live out your fantasies in the shower, man. Something like this can seriously muck up a living situation (and your relationship with your girlfriend).

If your girlfriend and yourself are ready to try a threesome the two of you should pick out the third party. In this instance, I am going to assume it will be a girl. First- your girl's friend with the boyfriend is out - unless you would like a foursome with her mate involved. That's another question with another answer. Honestly, would you let your girls have a threesome with another couple? It's doubtful. That doesn't mean you can't live your fantasy, though. You just need to find a nutural, unattached party. You both have the right to say no to any one the other picks out. Trust me, you will go through many vetoed girls before you settle on one your both willing to do this with. Don't try and convince her to be ok with a girl you pick, and vice versa. Things like that lead to regrets and often the end of a relationship. I will assume this is not your clever breakup plan. Talk extensivly with your girlfriend and find out what each others limits are. You may think this is a no brainer- think again. There are many things she may not want you to do, and many things you may not want her to do. With a MFF threesome though, it is more likely to be things she does not want you to do, unfortunatly. These need to be discussed, and aggreed upon beforehand, so there is no disuption in the 'moment' to spoil the mood, or ruin it for everyone. Be sure your third party is aware that this is a strictly sexual encounter. She will not be calling the next day to see of you want to go for coffee, or whatever. If anything she should hang out with your girlfriend for best chances at a second encounter, if thats what everyone is up for. Be doting on your girlfriend before, durring and afterwards. Make sure she is aware that this is something you wish to share with HER. Talk to her about how lucky you are to have someone like her. I'm sure you are aware the majority of girls would laugh in your face at such a request. (I am not one of them.) Communicate, Communicate, Communicate. I can not stress this enough. If either of you have bad feelings afterwards the two of you need to talk it out as soon as possible. Talking it out can keep a relationship together, where as not mentioning it is likely to split you up. Best of luck and have a great time!!!

you sir are a dick.

Arrange a group sex, all of you four will enjoy it.