2021 taxes shows credit for 3rd stimulus. 

Wait, we are doing 2020 taxes now, not 2021.  The Recovery Rebate Credit on your 2020 return is just for Stimulus #1 and/or #2.   But the IRS might deny you if your ex already received those payments   

The IRS is sending out Stimulus #3 payments in batches, and will take a few weeks to complete them all.  This one comes to you separately (and automatically), not as part of your 2020 tax refund.  You can check the status here:

Get My Payment | Internal Revenue Service (irs.gov)