Which one should I prefer: IIIT Bangalore ECE, IIIT Lucknow CSAI or IIIT Gwalior CSE?

CS and EC are very different branches. You need to decide which branch interests you more.

EC is about electronics and how the different electrical components communicate with one another. It involves a lot of Physics. It only involves a little programming. CS, on the other hand, is all about programming. It mainly uses logic and math. 

If you are interested in programming, CSAI from IIIT Lucknow is the better choice. AI/ML is in huge demand in the industry as most things will be automated in the future. Placements and coding culture are top-notch at all these colleges. Students from IIIT Lucknow and IIIT Bangalore perform well in competitions like ACM ICPC. 

All these colleges are good. You should make your choice based on your field of interest.