File an amended return to claim your third economic impact payment.
The IRS corrected returns that had the wrong stimulus amounts, but the service did not add stimulus payments if you did not claim them on your 2021 tax return.
If you used TurboTax in 2021 you can amend your return for free.
See: How do I amend my federal tax return?
If you used an accountant or other tax preparation software, you can buy and install TurboTax in your computer to amend your return,. You'll first enter your original data the way it was filed before starting on the amend process.
Go to File your 2021 taxes to buy TurboTax.