If you have a child born in 2021, you will need to claim the Recovery Rebate Credit to get the 3rd stimulus payment on your 2021 tax return.
The IRS has already started sending 3rd round stimulus checks based on 2020 returns, and had until December 31, 2021 to issue all of the Third Economic Impact Payments.
The Recovery Rebate Credit did not calculate a credit for my child born in 2020
According to the IRS: "Unlike the first two payments, the third payment was not restricted to children under 17. Eligible individuals received a payment based on all of their qualifying dependents claimed on their return, including older relatives like college students, adults with disabilities, parents and grandparents".
Generally, if you had the same number of qualifying children on your 2021 tax return that you also claimed on your 2020 return – even if they are not the same children – it’s likely a payment was already issued to you, for the full amount of the Recovery Rebate Credit as the Third Economic Impact Payment.
If you were issued the full amount of the Third Economic Impact Payment, you won’t be eligible to claim the Recovery Rebate Credit for your child born in 2021 even though that child is a qualifying child for the credit.