If you did not receive the second stimulus payment then on the 2020 federal tax return, after completing all the entries for the federal return, in the section for Federal Review you can indicate that the second stimulus was not received. This will put the Recovery Rebate Credit on the Form 1040 Line 30. The credit will be included in the federal tax refund or it will reduce the amount of federal taxes owed.
The third stimulus payment is sent directly from the IRS and is not part of the 2020 tax return.
Go to this IRS get my payment website for the third stimulus - https://www.irs.gov/coronavirus/get-my-payment
