Hey Nagaraj

I guess the rules vary for different borads and since you have not mentioned which boad you are from I will assume you to be from cbse. Candidates need to fill the CBSE Improvement Exam Form 2020 for the subjects in which they want to improve their scores be it only physics or all three (PCM).  As far as your question goes the - students can apply for one or more subjects as per their choices as mentioned in the CBSE improvement exam rules.