File a 2021 tax return. The IRS is no longer sending out stimulus payments so you'll file an original or amended 2021 return to claim the third stimulus payment.
It will be added to your refund as a Recovery Rebate Credit.
If you already filed those years, file an amended return. 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.
You can only prepare the current tax year online in TurboTax. To file prior year returns, buy the prior year programs and install them on your computer.
Go to File your 2021 taxes.
If you used TurboTax in 2021 you can amend your return for free.
See: How do I amend my federal tax return?