Hi

First of all you don't need 75% in 12th  to be eligible for jee mains as there's no percentage barrier to be eligible for JEE mains.

For example,

To be eligible for JEE MAINS 2020 , To be eligible for JEE mains 2020 for BE or Btech you just need to be either appearing for 12th board exam in 2020 or should have passed it in 2019 or 2018 with Physics, mathematical and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject.

So, you are still eligible for JEE Mains examination even though you have not got 75% in 12th

But yes,

To be eligible for NITs, IIITs, IITs, GFTIs one needs at least  75% marks in  class 12 if belong to general or obc category or at least  65% marks if belongs to SC/ST or  must be in the top 20 percentile in  class 12 examination.

Now , if you want to get into NITs, IIITs, IITs, GFTIs then you can go for NIOS on demand examination to fulfil the aforesaid criteria .

Thank you