What should I take, IT in IIIT Allahabad (5 years), ECE in IIIT Allahabad (5 years) or ECE in NIT Allahabad?

Among all the options you have mentioned, the one which you should actually consider is ECE in IIIT Allahabad for a duration of 5 years. 

The closing rank of IIIT Allahabad is better than ECE at NIT Allahabad and IT at IIIT Allahabad. ECE is best pursued at MNNIT Allahabad rather than IIIT Allahabad. But then I should also mention that IT is better for IIIT Allahabad than anywhere else. That's a fact.

If you have a knack for coding and it holds your concern, the environment that is accurate for a coding culture can be provided by IIIT Allahabad. 

Therefore, if coding and IT are your forte, a 5-year IT course at IIIT Allahabad is the one for you. The faculty there are experienced and carry specializations that guide you through your way. 

Look at the bigger picture which holds your future and then go for an aim. Choose such a specialization that ensures security to your future as well. 

But then, if you eventually want to settle on ECE, MNNIT is the right place.