Hmm In the end, its very much up to you dude but some things to think about are:
1. You only live once and in my mind try to take every possibly good experience thats on offer.
2. Think about the mindset of the other girl, I'm sorry ladies but as much as guys can be pricks, women can play some pretty horrific mind games to get their own way.
3. Everything might be great at the idea stage but it can be alot different when in practice. Someone might suddenly change their mind, it could even be you and then things become sour.
4. Your girlfriend can get jealous either mid sex or sometime after about something simple like "the way you looked at her" or "you've never done that with me" etc and that starts a rift between the two of you.
5. Is it really worth it? Is it something you personally desperatly want or is it just a gimmicky kind of thing that your not really to arsed about. Would you still want one even if she was against the idea.
6. From my "experience" its happend after being drunk and it can become quite a funny, laughable scenario, even silly. Its still a threesome but its nothing like you'd see in a porn film. Granted this proably was just the drink that made mine go a bit comical.
At the end of the day. It's up to you. This probably isn't the question an internet forum holds the answers to.

Talk it over with a mate who knows you both and can give you an informed external view on the situation. Though be prepared they may ask to join in haha