Foofa
For all you know it's your girlfriend this woman is interested in. If you don't like having a third wheel along just don't let her hang with you. On the other hand if you're looking for a threesome you might be able to make that happen.
Mou Allen
I have been the 3rd wheel quite a few times. Mainly because I enjoy the company of my friends, the other times I was just bored and had nothing to do. However, it sounds like maybe you are the one that likes this girl.
blackgrumpycat
What makes you think she is interested in you? She might be after your "chick".