I have already been garnished by the Hospital for medical bills. They have found two more bills (old ones) and are trying to garnish me again. Is this legal??

Yes, if you have various debts you can get 3, 4 or even 10 judgements against you, and each can get a order of garnishment, Now depending on state law, they may only garnish up to a max amount on all garnishments, but after one has been paid in full, another one will kick in.