The third stimulus payment is not taxable income for 2021. The system will only ask about it if you are eligible to reconcile what you received with what you should have gotten.
If TurboTax determines you are not eligible, either due to income levels, filing status, or residency requirements, it will not even ask you about the third Economic Impact Payment. If you received the money and it turns out you were not eligible for it, it is yours to keep and it does not need to be entered on your tax return.
