Hey Amit.

Yes, you can apply for admission in any 3 year law course even if your final marksheet is not here yet. You haven't mentioned the name of any college. But the common practice is that you will be given admission provisionally as per your other marksheets of previous years. And your admission will be confirmed after you have submitted the final marksheet when the result is out and it's ready. The last date to submit the final marksheet would differ from college to college.

Best of luck