Hi Ram,

I can understand that it gets taxing to prepare for all these crucial exams at once starting off with your boards, however, a persistent and methodical approach to your preparation can help you achieve your desired score. My advice to you would be to manage your study time in a way where you can devote ample hours to both your 12th class syllabus and JEE Main as the syllabus for both is the same. However, JEE Advance is a little different and many of your school modules are not a part of it. 

Therefore, my suggestion to you is to currently strengthen your grip over your basic concepts and then proceed to solving mocks for JEE Main. It's only a short apan's hard work with long term benefits. Just be consistent with your studies and you'll do great.