First, ratbastid is sitting next to me and he says he was mostly joking about the threesome idea. He's funny like that sometimes. Or...not funny....
Make a decision, dude.
If you're committed to your girlfriend, keep it in your pants.
If you're not committed to your girlfriend, break it off with her and (ratbastid says "bang her like a screen door
) go play the field honestly. No need to hurt an innocent third person who thinks you're being faithful.
Don't be the cheater guy women all complain about. Don't try to have your cake and eat it, too. Do the right thing: make a choice.