Take 6 chapters of a subject each day.
Spend half an hour on principles and formulas for each chapter.
Revise 30 problems of a chapter in an hour. It will be good if you marked difficult problems in your earlier study. Revise those problems now.
That way in 9 hours you can complete 6 chapters in a day.
You can complete all the three subjects before the examination.
I think this last 3 week preparation will add a lot to your earlier effort and make you do your best in the exam.
All the best.
Write tests conducted by your coaching institutes. If they tell you to read some chapters in more detail based on those tests, read those chapters. Remember it is time that you put in 12 to 13 hours effort for JEE. So you have lot more time to think your own ideas as well as to follow ideas of your coaching institutes.