I efiled my taxes yesterday and thought i did not receive a third stimulus check. i in fact DID receive it. am i able to change that information by amending my taxreturn?

The IRS knows that the 3rd stimulus payment was sent to you.  They will adjust your tax return accordingly by removing the credit and if you are still owed a refund this may delay the refund by several weeks or longer.

There is no need to amend the 2021 tax return.

No you cannot fix it now.   You just have to wait for the IRS to fix your mistake.   By answering incorrectly about the stimulus amount you received you put an amount on line 30 of your Form 1040.   The IRS checks that line.  They know how much they sent to you.   They will reduce your refund by that amount.   This will delay your refund so expect a wait.