When you file the 2020 return if you did not get the first 2 stimulus payments then that should have been indicated in the interview and the amount you are eligible for based off the 2020 income would be on the form 1040 line 30. By filing the 2020 return the third stimulus you are eligible for should be released automatically when the 2020 return is processed. Before you try amending anything take a look at the 1040 line 30 and the stimulus worksheet to see if you already got the credit and if not were you eligible for it at all.
