I forget to claim recovery rebate credit for my son who born on 09/2020. And already submit my tax return. What should I do? Also, 3rd round Did not included him either.

First, you need to make sure your tax return is fully processed by the IRS before making any changes to amend your return.  They may automatically adjust your refund if applicable. In the meantime you can check certain sections of your tax return and IRS data.  

1. Review the Recovery Rebate Credit worksheet included with your tax return file to see if the stimulus amounts are correct.
2. Check Line 30, Form 1040, to see if you the right credit on that line (including your newborn).
3. Compare your refund you actually receive with the information above.
4. Check Get My Payment - Select the button Get My Payment to see if your information is current based on the payments you did receive.  This is the information IRS has about your stimulus payments.
5. Once you review, and if you determine you have not received all of your stimulus you can amend (available later this month)

• How to amend (change or correct) a return already filed (and accepted at the IRS)
  • Review the information here to be sure you need to amend your return.
• Steps to amend my 2020 return