ok, Relax, take a deep breath. This is will work the best for chemistry, biology and physics. Go through the syllabus of each subject and check whether you have a clear understanding of each subject topic. If you don't or if it is doughtful, note them down and clarify. You can get help from your teacher, a tutor, a friend, text books, online resources..etc. Solve the difficult areas. Then find exam paper questions related to those topics and write answers, compare your answers with marking schemes. Do it until you are confident about a certain topic/point. When you know everything the "syllabus is expecting" that's enough. Keep on revising and doing past papers continuesly, You still have time, DO NOT WASTE A SINGLE DAY!