You have some of the weirdest misconceptions about martial arts I have ever read. There is no such thing as a purely defensive martial arts or a purely offensive one. You can not win with just offense. And what do you mean you need to train technique only? That's the hard part. That's the whole point of martial arts. Being in fit is secondary. Important but still secondary. So I have no idea what you want or what you're looking for. I think you've watched too many movies.

The only thing that will teach you something useful in that short space of time is boxing.

It's not the art that matters, it's how you train. You fight the way you train.

Kajukenbo or Tae Kwon Do.

Aikido is a purely defense martial art, and probably the best one for what you're asking for. Kenpo is offensive/defensive like most other martial arts.

3 months are very less for someone to learn martial arts.. but still you can try keysi fighting method (KFM). It takes relatively less time.

I think judo or jijutsu is good for what you want to do. Great exercise and provides plenty of practice.

I am going to college next year and I want to learn a martial art that will purely help me to counter attacks and defend myself. Don't get me wrong I'm not asking for a martial art that will help me in the streets best. I'm looking for something that will enhance mostly my defence. A defensive style I mean. Not something like Krav Maga which will teach me to aggresively attack and defeat my opponent or Aikido which has throws etc. Kenpo seems excellent but it requires a lot of time to master as far as I know. I need something that I can learn like in 3 months (I'm in very good shape so what I need to train there are techniques only) Which style can you offer me?