The third stimulus payment is not on your 2020 tax return. The third round of Economic Impact Payments will be based on the taxpayer's latest processed tax return from either 2020 or 2019. If the IRS has received and processed a taxpayer's 2020 return, the agency will instead make the calculation based on that return.
If you had a new child in 2020 and added your child as a dependent on your 2020 tax return the IRS will possibly make an adjustment after your 2020 return has processed. If not then you will have to claim it most likely on your 2021 tax return.
Please see How will the third stimulus impact me? for more information.