I filed our taxes checking that we did not receive our third stimulus check, but going back thru our records we have received all 3. Do I file amendment immediately?

No do not amend.   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.

No, you don't amend the return at this point, that might makes thing a bit messier. 

The IRS has your payments on record and will adjust for it. 

They will send the reduced refund (or a tax bill) with a letter explaining the change. 

You will have the opportunity to accept the change, or appeal. 

The return may take longer to be processed. All you can do is wait. 

IF YOU WILL NOW OWE, make sure you get that tax payment paid before April 18th. The payment can be sent in without an amended return. 

Ways to pay the IRS