Every year students choose PCMB to keep their options open, then carry five subjects through the two hardest years of school. Sometimes that is the right call. Often it is not, and the decision was made out of anxiety rather than interest.
PCM
Physics, Chemistry and Mathematics. The route to engineering and JEE, architecture, and most B.Sc. programmes in Mathematics, Physics and Statistics. Choose it if you are genuinely comfortable with Mathematics — comfortable meaning you enjoy working a problem, not merely that you score well in school tests.
PCB
Physics, Chemistry and Biology. The route to NEET and medicine, dentistry, pharmacy, physiotherapy, biotechnology and life-science research. Choose it if Biology holds your interest as a subject in its own right, not only as the path to a medical seat. The syllabus is long and largely retention-driven; interest is what carries a student through it.
PCMB
All five subjects. It does genuinely keep both doors open, and it is genuinely a heavier load — roughly a third more syllabus in the same number of months. It makes sense for a student who is undecided and academically strong. It is a poor choice for a student who is undecided and already struggling, because it usually produces mediocre performance in both directions.
One question worth asking
Which of these subjects would you still read about if there were no examination at the end of it? That answer is more reliable than any aptitude test, and considerably more reliable than what your friends are choosing.