I figure that after two years both of you should know if you would like to settle down or not. If you are so secure in your relationship that your GF won't develop feelings for someone else after engaging in a 3-way, then do what you need to do.
I had a friend who was in the very same predicament as you are, he was in a multi-year relationship with his college sweetheart, and they wanted to engage in the swinger lifestyle. I advised him not to do it, but he went ahead and did it anyway. Needless to say, that relationship flopped as soon as the sex was over.