The third stimulus check will not affect your 2020 tax return, but it may help your 2021 tax return. If you are eligible for more stimulus money than you receive with the advance payment, you may be able to claim the difference as a credit on your 2021 return that either decreases your tax liability or increases your refund. If you're eligible for less or received a stimulus check for the full amount of the Recovery Rebate Credit, you don't have to pay it back and it won't affect your return.