File a 2020 tax return to claim the 2020 Recovery Rebate Credit even if you don't usually file a tax return.
File an amended return to claim the credit.
To find the amount of the third payment, create or view your online account or refer to IRS Notice 1444-C, which will be mailed after sending the payment. In early 2022, the irs will send a Letter 6475 confirming the total amount of the third Economic Impact Payment and any plus-up payments you received for tax year 2021.