How do I claim my 3rd stimulus check? I've already filed my taxes for 2020, but was not given the choice to receive my 3rd stimulus check.

The third stimulus payment is not recorded on a 2020 tax return.

Go to this IRS get my payment website - https://www.irs.gov/coronavirus/get-my-payment

You don't. The Government is sending letters to all recipients stating you will receive the third stimulus Check or auto deposit within (IE. 7 days).  Its automatic to all whom qualify; IE. fall within financial guidelines.