If I deposited a check for $3,000 and took the cash but it turned out to be bad, what should I do?

Be prepared for the bank's collection efforts. You now likely owe the bank $3,000. Failure to repay this will likely lead to fees, fines, interest, negative credit reports, and possibly legal action. I would recommend you repay the money as soon as possible. 

If someone sent you a bad check, you should also try to receover against that party. Depending on the reason it was bad and what the check was for, you could have a variety of causes of action.