On the 2020 return only the missing first and second stimulus payments were allowed as a credit NOT any part of the third stimulus payment. NOWHERE in the interview were you asked about the third stimulus amounts.
If you said you did not get all the first 2 stimulus payments you were entitled to in advance then those missing amounts could be a credit on the 2020 return so if the IRS disagrees with the amount you listed on the return you will get a letter like you did. Your only option is to pay the amount on the notice and then if you did not get the amount you can have a trace put on the check ...
Please see How do I request a payment trace to track my first or second Economic Impact Payments?
