The round 1 and round 2 stimulus payments are reconciled on your 2020 tax return. Any amount of stimulus payment will be reported in a letter number 1444 or 1444-B sent to your from the IRS. If the IRS believes they issued a payment, you can't claim a rebate on your 2020 return because the IRS will automatically remove it. You have to trace the missing payment instead.
See question F3 on this page.
https://www.irs.gov/newsroom/2020-recovery-rebate-credit-topic-f-finding-the-first-and-second-econom...
The round 3 stimulus will be reconciled on your 2021 tax return. Although we don't have the 2021 instructions yet, it is likely the procedure will be the same. If the IRS issued a stimulus payment, they would send you a letter number 1444-C. If the IRS sent a payment but you didn't receive it, you will have to initiate a trace.
