Should I have a three some with my friends?

Here's the deal. So I have two friends that are in a relationship. I'm closer with the guy - we go home on public transportation together, and we talk about a lot of stuff, whatever. The girl I am also good friends with, she makes jokes about she and I having sex together and I go along, and one time she fondles my boobs and I was cool with that (she's just that sort of person).

So the guy and I were talking (we talk about their bedroom life a lot, there are a lot of entertaining stories, and interesting facts about the act that I hadn't known beforehand. Of course we talk about other things as well, but both of us basically are incapable of feeling awkward so we talk about what they do together) and he goes "Oh by the way, *girlfriend's name* wants to to know if you'll have a threesome with us."

I wasn't surprised by his asking, I knew she had wanted a threesome for ages and that he was cool with it, I wasn't even surprised that she chose me, since we all trust each other and are very relaxed around each other. I didn't answer him then, but I'm thinking now.

First off, it sounds like so much fun. I haven't gone that far before, but they have many times, and so they know what they're doing. And for obvious reasons, that sounds great. In fact, when he first told me, it sounded like a great idea and so much fun. I am very comfortable with that being my first time, because I trust them like that.

However.

My friendship with the guy is very important. He is loyal (the most important quality, as far as I am concerned), honest, trustworthy, smart, and he gets my sense of humor. If he made weird jokes about the experience afterwards, I would feel uncomfortable, and I would definitely get really mad if he ever made jokes about how I did badly. I could tell him to stop it, which would work that out, but I do not want to lose his friendship on account of this. I don't want it to be awkward because we did something like this. At the same time, it might enhance our friendship - we will have gone to the ultimate level of friendship, and be able to trust each other completely. Yes, I do think this is very possible.

What ending will be more likely? Could it hurt my friendship with the girl? What should I do?

Note: We go to a college with fewer than 200 people (yes, colleges that size exist), so we will still see each other quite a lot, even if we try to avoid it if the worst happens. We are all 18, but I am transferring to a different school next school year.