The guided interview process has a step in the "Federal Review" section where it asks how much you received for your two stimulus payments. You would enter 0 if nothing was received.
You can see how much the IRS though you got by registering with the IRS web site where you can view the actual stimulus payments: View Your Tax Account | Internal Revenue Service (irs.gov)
Which one didn't he get? Only the first 2 go on your 2020 return. Don't worry about the 3rd one yet.
When you go through the federal review tab it will ask how much you got if any. Enter a 0 in the box if you didn't get one. Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus."
After you have entered your tax information and are ready for the Federal Review, it will be the first question.
If you are trying to trigger it, you can go to Other Situations, answer any questions and click Done. The next automatic selection is Federal Review and it will ask about payments received.
If you didn't get one or both of the Stimulus payments or qualify for more it will be the Recovery rebate credit on 1040 line 30. It either adds to your refund or reduces a tax due.