How to tell a blind date I'm not interested?

So I'm going on a third date with this girl that I'm not interested in. It started as a blind date, one of my girl friends suggested it so I went along with it. She's a close friend of hers. I like hanging out with her but I'm not attracted to her romantically at all. Now, I know I should have just been direct after the blind date, but I didn't want to be rude. But I'm a nice guy and a sucker so I told her after the first one that we should hang out again. Also, I figured a second date to get to know each other wouldn't hurt. We did, had dinner and some drinks. She then got back to me saying she wants to hangout again. So now when we hang out again I want to nicely and diplomatically let her know I'm not interested. She seems interested so I don't want to hurt her feelings. We're not teenagers btw, so mature words are needed here. Thanks.