The first two stimulus checks (EIP1 & EIP2) should be included in your 2020 refund (or applied to your tax due) as the Recovery Rebate Credit (RRC), unless you're ineligible for another reason (or your first check was offset for child support, the only check that applied to). If you tell TurboTax you didn't receive either of the first two checks, it will compute the RRC and apply it to your return much like any other refundable credit.
The third check (EIP3) is just now going out (it hit my bank this evening but I'm probably one of the first), so I can't say for certain about that. However, I expect the IRS will issue additional third checks at some point to those whose returns were processed after this weekend; there may also be another IRS "non-filer portal" similar to the one that was available for EIP1 (which also qualified those people for EIP2 & EIP3).
