Hello,

Yes, you are eligible to appear for Combined Defence Service Entrance Examination (CDSE). Please check the Education qualification requirement for Each Academy

• I.M.A. and Officers’ Training Academy, Chennai: A candidate must have passed Bachelor's Degree from a recognised University.
• Indian Naval Academy: A candidate must have passed Bachelor's Degree in Engineering from a recognised University.
• Air Force Academy: A candidate must have passed Bachelor's Degree of a recognised University (with Physics and Mathematics at 10+2 level) or Bachelor of Engineering.

It does not matter if you have passed in B.E/B.Tech after Diploma in Engineering. As you have requisite Education Qualification you are eligible to apply for CDS provided you must also fulfill the age criteria.