The 3rd stimulus payment IS based on the 2021 AGI. The stimulus payments received in 2021 were an advance credit based on the 2020 AGI since that is all that the IRS had to go one to get the payments out to taxpayers.
Go to this IRS website for the third stimulus payment information - https://www.irs.gov/coronavirus/third-economic-impact-payment
Calculating the 2021 Recovery Rebate Credit - https://www.irs.gov/newsroom/2021-recovery-rebate-credit-topic-e-calculating-the-2021-recovery-rebat...
The 2021 Recovery Rebate Credit worksheet - https://www.irs.gov/pub/irs-pdf/i1040gi.pdf#page=59
