IIITDM Kancheepuram is an institute of national importance and it has an advantage over the other two institutes as both the institutes are IIITGs in PPP Mode. The bill to declare PPP mode as institutes of national importance is still pending in parliament.
Here are a few pieces of information about the CSE/IT in the campuses mentioned in the question.
IIIT Guwahati:
The institute started in the year 2013 and has a strong CS department with a good number of experienced faculties.
IIIT Sri City:
Both the colleges have their advantages and disadvantages. You have to analyze which is better for you to pursue B.Tech in CSE/IT as per your parameters. Good luck.