If you want to join Indian Armed Forces after doing a Diploma course, you must think about that plan again. It is better to go for Graduation Degree rather than going for Diploma course. There are many Degree courses that lasts 3 years only, just like the duration of Diploma courses. One may appear for exams like CDS (Combined Defence Services) as well as AFCAT after Graduating and thus make it to the Indian Army or IAF (Indian Air Force).
Also think about the post retirement scenario- Degree courses such as Engineering and B Sc will provide ample job opportunities even after one retires from Armed Forces. I do agree that Diploma courses are good too but the above mentioned Degree courses holds more value!