Hello,
You need to revise firstly the NCERT BOOKS for important concepts.Then practice previous year's questions that will help you to understand the pattern of the exam.Follow a proper timetable and allot equal time to each subject.Concentrate only on studies and don't get distracted.
Good luck.

You are left with only three months so first of all you need to create a particular time table and you must strictly follow it you have three months and it is sufficient to revise syllabus but while revising keep in mind that you just don't prepare for your exams but also for learning therefore try to go into as much detail as possible.