Hello there!!

1.If you want to join the army after class 12th, I guess you are talking about NDA.

For joining the Army wing of NDA you can have any subject in class 12th. So, yes PCB will work in NDA. However for Air force and naval wing you must have Physics and Mathematics in class 12th.

2.For BSF and CRPF also there are restrictions regarding subjects at 10+2 level for some particular post which require Maths and Physics. So yes you can join BSF and CRPF also but at constable level.

3.In BSF and CRPF with PCB you can join at CONSTABLE level. For Army you can join NDA army wing.

Hope this helps