Hello Aspirant .

Just taking PCM(Physics, Chemistry, Maths) won't make you eligible to give IISER examination. For that you'll have to score atleast 75 percent in your class 12th examination (if you're from General, OBC category)

If you're from SC/ST/PWd category then it is mandatory for you to score atleast 65 percent in your class 12th board examination. Only then you'll be eligible

Best of luck !