Forget about 2018 and 2019. Those were just used to send you the first 2 Stimulus payments in advance of 2020.
If you did not get one or both of the first 2 Stimulus payments you can claim the missing amounts on your 2020 return. It will be listed on your 1040 return on line 30. Check there for it. It either adds to your refund or reduces a tax due.
Don't worry about the new 2021 3rd one yet. Only the first 2 get reconciled on the 2020 return.
