Full form

IIT - Indian institute of technology

NIT - National institute of technology

IIIT - Indian Institute of Information Technology

Government Funding

IIT - Heavy government funding

NIT - Moderate government funding

IIIT - Moderate government funding (since IIIT Act 2014)

Focus:

IIT - All round development and all engineering fields

NIT - All round development and all engineering fields

IIIT - Razor sharp focus on IT

General Perception :

IIT>NIT>IIIT. There are 23 IITs , 31 NITs and 4 government owned IIITs.

IITs use Jee Advanced rank while NITs and IIITs use Jee Mains rank for admissions.

There is a joint counselling for all of them.

IITs have very large campuses , NITs have moderately large campuses and IIITs have small campuses.

The top IITs , NITs and IIITs have quite hardworking students.

Admission to all of these is merit based.

Unlike private colleges their fees are not exorbitant.

Hope this helps you.