If you have a child born in 2020, you will need to claim the Recovery Rebate Credit to get the 1st and 2nd stimulus payments. If you haven't filed your 2020 return yet, it is likely too late to get a third check for babies born in 2020. The IRS has already started sending 3rd round stimulus checks based on 2020 and 2019 returns. You may have to wait until you file your 2021 return next year to claim that money on your return.
The Recovery Rebate Credit did not calculate a credit for my child born in 2020
In 2020, you had a new baby and one of the children on your return (that qualified for the stimulus) turned 17: According to the IRS website (Question D18),
"The Economic Impact Payments were advance payments of the Recovery Rebate Credit. To issue the Economic Impact Payments as quickly as possible, applicable laws allowed the IRS to use your 2019 tax information. The IRS used your 2018 tax return information for the first Economic Impact Payment if your 2019 tax return was not on file.
Generally, if you had the same number of qualifying children on your 2019 tax return that you’ll have on your 2020 tax return – even if they are not the same children – it’s likely we already issued you the full amount of the Recovery Rebate Credit as two Economic Impact Payments.
If you were issued the full amounts of Economic Impact Payments, you won’t be eligible to claim the Recovery Rebate Credit for your child born in 2020 even though that child is a qualifying child for the credit."