In order to pursue three year LLB, you must have completed your graduation and if you want to go for integrated LLB i.e. 5 year LLB programme, you can apply after completing 12th class.
Completing a diploma isn't an eligibility criteria to pursue LLB, hence, the duration of the LLB course remains as stated above even after completing diploma.
Hope this helps. Thank you so much.
In case of any queries, feel free to ask. Good luck :)