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National Law Universities only offer integrated law courses for a duration of five  years,  admission is done through CLAT which has the eligibility criteria os passed 10+2 from a recognized board with 45% aggregate, in case of reserved categories there is relaxation of 5%, if you want to go for three year LLB, the eligibility criteria is passed graduation in any discipline from a recognized university with 45-50% aggregate depending upon the norms of the university, some of the top entrance test are DU LLB, BHU UET, LAT by Allahabad University, there are also state level three years law entrance test such as AP LAWCET, KLEE, TS LAWCET, MAHCET LAW to name a few, check out the colleges which offer 3 years LLB at https://law.careers360.com/colleges/list-of-llb-colleges-in-india