Hello ,

No worries, if you don't get enough percentage in your 12th class. As , there is no criteria for accounting your grade of class 12 in JEE. The only thing you should be focusing of your preparation .

One things , if you had started your preparation , after 12 and you are not confident about your preparation  then  may be chances are low to get an IIT (mainly depends on your smart work and consistency) .

But you you should mainly focus of JEE MAINS as , the exam is not that tough and with max syllabus coverage and not going in depth in concept you can clear the exam and can get into any GOOD NIT ,that is far better than going for BCA or BSC.

If by any chance , you are not getting selected , then you can purse BCA ,  as computer science as IT industry is in great demand  with less supply of  good computer graduate.

Don't waste your time for 3 attempt in JEE, as it only going  to take your one more YEAR.