Withdraw and tell them to give your spot to me! =)
In all seriousness, I'm not sure how much $ Stanford gives out. I'm inclined to believe that HY might listen to money offers, but then again, they are H and Y. I think either way, all three schools should give you similar aid packages, although some schools might have better private loan options/etc. If you are low income, then some schools might have better grants, or if you want to do PI, then LRAP programs might come into play. I would research a little bit, and see, but I don't see Stanford throwing you a lot more $ than H and Y unless they have significantly different finaid policies (which I don't believe they do).