If you did not receive your first and/or second stimulus payment or if the amount received is not correct or if you did not receive the stimulus for a dependent child under age 17, you can claim it on your 2020 tax return as the Recovery Rebate Credit.
In TurboTax Online, to claim the Recovery Rebate credit please do the following:
Please see this TurboTax FAQ for instructions on how to amend your tax return in TurboTax.
Why do you think you need to amend?
If you qualify for a rebate based on your income and number of dependents, Turbotax automatically adds it to your tax return. You have to take affirmative steps to remove it, by telling Turbotax if you already received the round 1 and round 2 stimulus checks. If you do not remove the rebate, it is added to your tax refund on line 30 of form 1040. It is part of your refund, it is not paid separately.
Check line 30 on the tax return you filed. Did it already claim a rebate? If not, then you may not qualify, and amending will make no difference. We would need more details to help you figure this out.
The round 3 stimulus is not part of your tax return, it will be paid separately.