U have to strong ur all 3subjects first thing.

For chemistry read theory a no. Of tymes.

Maths formulas and concepts must be remembered.and need practice regularly 

Physics theory and concepts must be cleared and need practice.

U have to study regularly till end never loose hope.

Study min. 6-8 hours daily 

I’d advise you to start from NCERT books. Read NCERT textbooks thoroughly. Do the practice questions, exemplar questions. You should know them inside out. If you are from CBSE board, this will also help you in your boards. Even if you are not from CBSE board, it’ll help your board preparation because most of the syllabus would be a repetition. There might be only few extra things which you need to study. Maybe different experiments and all. Now for a score 250+ in Mains, once you finish NCERT books you should practice questions. Lots of practice! Start chapter-wise and later you can move to complete mock tests. Get to know your weak areas. Work on them by revisiting the books. Repeat the exercise until you get better. When you practice, ensure that you do them accurately. Mains is one exam where getting one question wrong might lead to an “effective 5 marks” deduction. This is not to scare you but to make you aware. Practice solving questions quickly but correctly. Avoid silly mistakes. (It’s difficult, but minimise it to the most). You have 3 hours and 90 questions. You can spend a cool 2 minutes on each question. Sometimes you might have to take short-cuts. Option elimination may sometimes do the trick. This is not always effective

Attempt questions only when you are sure. Consider this scenario, One guy attempts 75 out of 90 questions and gets 5 wrong leading to score 275 and then another guy attempts 85/90 and gets 15 wrong scoring only 265. Both got same questions right but number of incorrect questions were the deciding factor. I hope you get the point.

Hope this helps and all the best!

Thanks!