If. I received the 1st and 2nd stimulus payment by direct deposit, why would I receive my 3rd payment by mail?

The IRS did not have had your correct information on file for whatever reason. Something may have gotten lost over there in all the updates of banking information.

The IRS said it will use data already in its systems to send the third stimulus payments. Taxpayers with direct deposit information on file will receive the payment that way. Those without current direct deposit information on file will receive the payment as a check or debit card in the mail.

It wasn't worked out that way. I know several people who received one or two stimulus payments by direct deposit to a bank (not to a temporary debt card) and the IRS has sent them a check.