To receive any missed round one and two stimulus payments (1200/600), you need to file a 2020 tax return to claim a Recovery Rebate Credit on line 30 of that return. The information on your tax return will be used to qualify you for the stimulus payments.
Generally you need to be a US citizen, with a valid social security number, have an AGI less than $75,000 and not be claimed as a dependent on someone else's 2020 tax return.
Be sure to click the "Federal Review" tab when completing your return in TurboTax to answer the stimulus questions.
Here is a link with more information on the Recovery Rebate Credit.
Filing a 2020 tax return will also establish your eligibility for the third round payment (1400). Once your return is processed the third round payment will be sent separately but via the same method you indicated on your 2020 tax return to receive your tax refund (check/debit card, direct deposit).
Here is the link with information on the third round.
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