I have scored 75% in 10th, 58% in 12th and 58% in grads. what should be my CAT score to make it to top three IIMS??

I am assuming you are from general category. Based on your overall academic profile, you will need to score atleast 99.98 percentile to get a call for the next round.

All the best!

You require more than 99 percentile to get admission into top three  IIM. It also changes every year. they give weightage to your past academic records, graduation score and work experience. On the basis of that percentile shortlisting is done.

You can use below CAT pathfinder tool also to check score required to get all the IIMs.:

http://www.bschool.careers360.com/pathfinder/cat

All the Best!1