If your 2019 income was too high for you to be eligible for the 3rd stimulus check then you will have to wait for the IRS to use your 2020 income. They are sending out those checks in phases. There is no place on the 2020 return to enter anything about the 3rd stimulus check so you will just have to check on the IRS site to see if they are going to send it. If you do not get it this year you can use a recovery credit on your 2021 return next year.
You can use the IRS Get My Payment site to see what is expected for your 3rd stimulus check:
https://www.irs.gov/coronavirus/get-my-payment
https://www.irs.gov/coronavirus/get-my-payment-frequently-asked-questions