I think it's applaudable that you don't want to string someone along. But how do you really know if it's not working after only 3 dates? Perhaps the guy hasn't really gotten a chance to show you how funny, clever, smart, responsible, etc., he can be. I know that there are some that would disagree with me, but sometimes people can grow on you.
I think because you've been lead on by guys in the past, that maybe you've become a bit severe about trying to make sure you don't do the same to others. I wouldn't worry so much about it. There is no reason to not go on a fourth, fifth or sixth date with a guy who might have potential even if you don't feel the chemistry. Like Coco said, just make sure expectations are met and that he knows you want to take things slowly.