All these are India's premiere training institutes , the point of difference lies between there mode of entry , type of commision and age limit
IMA DEHRADUN
Mode of entry : Direct and TGC
Age limit : 19 to 24 and 20 to 27 years resp
Type of commision :permanent

OTA CHENNAI
Mode of entry :ssc tech and ssc non tech
Age limit :19 to 25 years
Type of commision :short service , can be extended

OTA GAYA
Mode of entry : 10+2 (Pcm) based on marks
Age limit 17 to 19.5
Type of commision: permanent.
Hope it helped.