<p>It could very well hurt future applicants from your school. If you are not yet 18 (or weren’t when you signed the letter) it is impossible that it could be interpreted to be legally binding, and even if you were that seems really unlikely.
And ignoring any moral obligations, I’d choose Yale over Princeton and Brown any day. I wouldn’t treat it as a binding agreement when you make your decision (as that could lead to regret later on) but I do hope that you make the right choice and choose Yale in the end.</p>