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)

No the new 3rd Stimulus is not on your 2020 return.  It only includes the first 2.  How old are your children?  If they are under 17 you should get all the amounts on 1040 line 30.

For #1  1200 for you plus 1500 for 3 kids

For #2   600 x 4 people 

Total 5100 on line 30.

When you finish the Review section it will ask how much you already got.  Enter 0 in the 2 boxes.

When you go through the federal review tab it will ask how much you got if any.  Enter a 0 in the box if you didn't get one.  Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus."

After you have entered your tax information and are ready for the Federal Review, it will be the first question.

If you are trying to trigger it, you can go to Other Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.

If you didn't get one or both of the Stimulus payments or qualify for more it will be the Recovery rebate credit on 1040 line 30.  It either adds to your refund or reduced a tax due.

Before filing,  You can preview the 1040 or print the whole return

https://ttlc.intuit.com/community/accessing/help/how-do-i-preview-my-turbotax-online-return-before-filing/00/26160