Don't worry about the 3rd one now. To get the 1st and 2ne Stimulus you have to file a 2020 return.
You need to file a 2020 return to claim the missing Stimulus payments. It's too late to be based on 2019 or 2018 returns. If you don't need to file a 2020 return then enter $1 of fake interest to efile.
See IRS Question Q A2 here
And see Question Q A4 says your return has to be complete and correct and include all your income and dependents, etc.