it is very possible that the 3rd stimulus was issued and you forgot / lost it or it was lost in the mail
1st thing to do: review your 2021 transcript at the IRS website and see if you there is a line item reflecting the 3rd stimulus - it will be in multiples of $1400.
if it is NOT there, then you'll have to amend as described by others in this thread
if it IS there, then the payment was lost and you'll have to complete form 3911 No amnendment is necessary as the IRS will be unable to help you.
https://www.irs.gov/pub/irs-pdf/f3911.pdf
