I told this girl I liked her and her response was 'thank you :) <3'?

I think her reply was a very polite and lady-like way of saying that she likes you back. She was probably shocked and was blushing, and that's all she could muster. It's a hard thing to react to without seeming TOO excited! :) The heart and the smile do not say rejection to me. Try asking her out, as you have absolutely nothing to lose. I don't think she'll say no, but if she does, at least you don't see her very often anyways. Good luck! :)