I am unable to attempt jee mains( I does not fill any application form ) in 2020 and I am in class 12 is I am able to attempt jee in 2021, 2022 and 2023

Dear student,

In JEE Mains you can give a total of three attempts and your first attempt is counted as over in the passing year of your class 12th.

It doesn't matter that you have given JEE mains or not in your class 12th passing year, your first attempt will still be counted as over.

Now as you are currently studying in class 12 then you cannot give your JEE mains 2020 exam in any way because you haven't passed your class 12th yet.
You can give in the passing of your class 12th passing year, i.e, JEE mains 2021 and after JEE mains 2021 you can still give two more attempts.
That means after jee mains 2021, you can give JEE mains 2022 and JEE mains 2023.

So you have all three attempts left currently, and if you give JEE mains 2021 then your first attempt will be counted as over, also it doesn't matter that you have given Jee mains in 2021 or not still your first attempt will be counted as over.

In simple words you can give JEE mains three times.
First time - in your class 12th passing year.
Second time - the next consecutive year of your class 12th passing year.
And third time - the alternate year for class 12th passing year.