I have completed a 3 year Diploma course in automobile engineering. Am I eligible for MBA or not?

You need to be a graduate to do MBA. Only Diploma is not sufficient.

After Diploma, you cannot directly take admission for Master's degree. You need to do graduation first. You need to qualify any entrance test, such as CAT / MAT/ XAT/ NMAT/ CMAT/ GMAT/ NEST and should also have minimum of 50% to 55% in your Bachelors. You can do MBA in any specializations of your in

You are not eligible for seeking admission in MBA as you have not completed graduation degree which is the eligibility criteria in majority of the universities in India for admission in MBA programmes.