Ok ... first the third stimulus would be released after you filed the 2020 return and it was fully processed. If you have moved since you filed you need to mail in a form 8822 to update the address on file. If for any reason you don't get the third stimulus by the end of the year then it will be a credit on the 2021 return you will file next year. https://www.irs.gov/pub/irs-pdf/f8822.pdf
Now did you file a 2020 return and fail to include ALL your income ? If so you need to amend the 2020 return to add the missing money :
To see how to amend your taxes, click here:
Contact the employer to get another copy of the missing W-2 form sent to you.
The IRS is not issuing any more round 3 stimulus checks. If you are eligible and did not receive a check, you can claim a rebate on your 2021 tax return.