Girlfriend threesome with friend?

So my friend and I were chilling in my room one night. We were both pretty wasted. I was texting & snapchatting my girlfriend throughout the night.

Well I was opening her Snapchats and he was looking at them with me (I let him, I didn't mind) and she all of a sudden started talking a little dirty. And one of the ones she sent said "we can have a threesome." We both were just kind of shocked when we read it and kind of laughed it off. A few minutes later she kind of brought it up again & I told my friend like "bro you know she meant me you and her". And he just nodded and was like "I'm down"

Should I even entertain this idea or brush it off. She hasn't brought it up again but if she did one night (like three days ago) then it probably means that she does want it, right? This guy is one of my best friends. Neither of us have done a threesome before. What should I do lol thanks