Hello student ca is different completely different in comparison of b com and FIA,as there's no college or attendance thing in CA So you can do both at the same time as you don't have to study a lot for Bcom..and after completing your class 12 you can directly apply for CPT (first stage of CA)

After completing CPT you need to clear INTERMEDIATE and then 3year of articleship and in the end FINAL.

CA is a 5year course if you clear all your exam in first attempt.

Feel free to comment if you have any doubt

Good luck