Seeing this stance brought up multiple times in this thread has me question just how some people think and how much confidence they have in their relationships. My girlfriend was the one who brought up threesomes casually ages ago and we eventually went through with it and had a good time and now we're still together and trying different things in our sex life (no we didn't move on to orgies, etc, we have no interest in that).Go for it but I wouldn't view the person as LTR material if they brought up threesomes.
Being confident about your relationship is an important factor here and just because they're sexually adventurous doesn't mean they're not relationship material, a threesome is a good way to judge this and what most would consider a testing ground along with basic stuff like handcuffs, etc.
If you're nervous, that's one thing, but all this cheating talk, etc, makes it seem like a lot of you have had bad experiences. The most important thing here is to TALK WITH HER, that is the key thing, you're not having sex with us, you're not in a relationship with us, you just need to be completely open with your partner about your thoughts on this, how you feel about each other, how strong and confident you think your relationship is, etc, because it could very well just be that she's happy with you and thinks the two of you are strong enough to handle a threesome and have fun with it.