I fail in physics, bio, and maths in 12th HSC board exam, so do I have to give all exams of all subjects in exams?
No, you can appear for the subjects you have failed in this year. HSC conducts improvement exams in July. Apply for the subjects you have failed in and reappear for the exam. If you do not wish to give the exam this year, then you have to appear for all subjects next year. Furthermore, contact your school and teachers, they can give you best advice.