No...you were not eligible for seat to NIT,IIIT,IIT...Coz the minimum % of 12 board exam is 75% to be eligible for seat to any IT

Good , u are eligible as they are expecting a mimimmi score of 75% . 

If u score good level marks in jee main u can get admission in nits.

Check the Jee main website for further details.

Best of luck

The eligibility criteria of JEE Main 2018 does not specify any pass percentage for appearing in the exam. This is because the exam is also an eligibility test for admissions to B.Tech or B.E programmes in states like Madhya Pradesh, Odisha, Nagaland, Haryana, Uttarakhand, Punjab, Chandigarh etc and the percentage required by students in class 12 varies from state to state.
So you are eligible.
Please visit following link to know more.
JEE main eligiblity criteria
All the best.