Send a tactful email saying what you just said, but give number amounts, and you may or may not have to add the names of the other two schools; its moot since all schools can see all you acceptances at this point. Better yet a phone call will do.
Negotiate. It's a business move. On your part and their part. Give facts, state them clearly and don't sound whiney. Put your big boy/girl pants/dress/suit whatever on and ask if there is anyway they could match or give a better offer than school xyz. Be polite, thank them for taking your call, say yes ma'am and yes sir and you'll be kosher.
Congrats on being in this position. Have fun with it.