The ONLY EIP payment you are asked about on your 2021 tax return is the 3rd one---the one for $1400 which was issued sometime after mid-March in 2021. You do not enter information about any other stimulus checks on a 2021 tax return. You will be asked in Federal Review if you received the $1400 payment.
The first EIP --or stimulus check--- was issued in 2020 It was for $1200 for adults and $500 for a dependent under the age of 17. The IRS used information from either your 2019 or 2020 tax return to issue those checks so they could send them out in a hurry.
The 2nd EIP or stimulus was issued in late 2020 or early 2021 and was $600 for adults and $600 for dependents under age 17.
If you did not receive the 1st and/or 2nd stimulus check you can get it by filing a 2020 tax return and using the recovery rebate credit in Federal Review. It will end up on line 30 of a 2020 tax return and will be added to your refund (or reduce your tax due).
The 3rd EIP--or stimulus check-- was sent out in mid-March or later in 2021 and was for $1400 per person--adults and dependents no age limit.
If you did not get the 3rd EIP payment you can get it by filing a 2021 tax return and using the recovery rebate credit in Federal Review. The credit will go on line 30 of your Form 1040 and will add to your refund (or reduce your tax due).
When you get to the review of your federal taxes you will receive a question asking if you received the economic impact payment and you will also be asked how much you received.
This is not claimed on your return as income, it is simply reconciling with the IRS that you did receive it.