We did receive the 3rd stimulus payment and mistakenly indicated that we had not. How do I correct that?

Did you already e-file?  If you filed then do nothing.

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.