Yes you can file a 2018 and a 2019 return. You have to file each year separately. BUT it won't help you get the Stimulus payments. You now have to claim any missing Stimulus on your 2020 return.
Forget about 2018 and 2019. The Stimulus Checks are really based on your 2020 return we are filing in 2021. So any amount you already got will be reconciled on your 2020 return line 30. If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return. But the good news is if they paid you too much they won't take it back.
For 2020......
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.
If you are a dependent or your 2020 income is too high you won't get prompted for the Stimulus section since you can't qualify for any more. And you don't have to enter what you already got and don't have to pay it back. 1040 line 30 is only if you qualify for more not what you already got.
