You do answer NO---you did not receive a stimulus check. And your mother did not receive a stimulus check for you since parents only got stimulus checks for children under the age of 17 at the end of 2020. When you complete the recovery rebate credit section you can say that you did not receive either stimulus check and the stimulus amounts will be added to line 30 of your 2020 tax return; then it is included in the refund amount that ends up on line 34.
The 3rd stimulus check is a separate check and is not entered anywhere on your 2020 return. You should get it later.
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