2020 tax return isn't filed and processed by the time the IRS starts processing third stimulus. I wasn`t eligible 2019 taxes but I am eligible for 2020. Please, advise

not to worry; worst case is that you'll get the $1400 back as a Recovery Credit on your 2021 tax return next year as long as you are eligible in 2021 

File your 2020 tax return as soon as you are able. The IRS has indicated that they will continue to process and pay round 3 stimulus payments as tax returns roll in. It will be paid separately from your tax refund if you are due one, and it will probably come a little after the tax refund.

Thank you