Yes, you are eligible.
This is because,
As per eligibility criteria, to be eligible one should have minimum qualification is 10+2 or equivalent from a recognized board. Students appearing / appeared in 10+2 can also apply.
Now, 3 year diploma course after 10th is equivalent to 10+2. So being in last year of your diploma, you are definitely eligible for you UGAT entrance examination.
YOU CAN CHECK THE OFFICIAL WEBSITE FOR THE SAME, the link is provided below