Hello student student I'm sorry to inform you but as diploma is not considered as a undergraduate course so you can't apply for 3year LLB course as its only for students who have passed their undergraduate of minimum 3year duration in any discipline. So you need to apply for 5year LLB for.
You need to appear in CLAT (Common law entrance test) to get admission in India's top law college. Form will available in first week of January and exam will held on 10of May 2020.

Feel free to comment if you have any doubt
Good luck