Should i study aeronautical engineering for 3 year or 4 year because i have both diploma and 12 with science

Hi Shrideep,
Firstly I would like to say, Aeronautical Engineering is the best course even though it is considered tough. For you or anyone I will always recommend a 4 year B.Tech program in Aeronautical Engineering. This is the best after your 12th and since also you have a diploma. B.Tech offers you the detailed study of the subject and doing this will also increase you chance's for getting hired in some the prestigious government institutions like ISRO, DRDO and HAL. So instead of 3 year I will recommend a 4 year course B.tech in AE.
GOOD luck