yes, Column A of the Amendment MUST start with the Tax Return as adjusted by the IRS. Column A is not what you submitted to the IRS originally.
first, be sure to save the *.pdf file that reflects what you submitted to the IRS. You'll want to save that for your records.
Then open up the file for your tax return, and re-save it with a different name, say, "2021- IRS adjusted file"
Adjust the questions concerning the 3rd stimulus so it matches the adjustment that the IRS made. Do you know EXACTLY how much the IRS adjusted and is that the only adjustment? Look at Line 30 of Form 1040; this is where the stimulus / Recovery Rebate changes will be reflected.
Once complete, save the file. Then re-save it with a different name, say, 2021- Amendment #1.
Once you are confortable what you are looking at on the screen matches the IRS (post the IRS adjustment), you can begin to amend. Column A should reflect what the IRS has and not what you originally submitted.
you can review your transcipt if necessary :
https://www.irs.gov/individuals/get-transcript
