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. 

• Prof Gautam Barua who is the director of the CS department at IIIT Guwahati was also the director of IIT Guwahati from 2003-13. With his immense experience, he aims to build the department strong. 
• Coming on to the curriculum of CSE/IT, it is well designed to suit the growing need of the industry. The intent here is to complete all the core subjects in the first 6 semesters so that students can choose the type of industry they want to get into. Like in the 4th semester, there is a subject called Computer Network which is generally a subject of 5/6th semester in other institutes. 
• In the CS department, IIIT Guwahati has come up with the Ph.D. program as well. 
• There are many advantages to join IIIT Guwahati but only a single disadvantage is there i.e. Location. But if you love to travel and nature, Guwahati is a place for you. 

IIIT Sri City: 

• The number of faculties at IIIT Sri City is nearly half as compared to IIIT Guwahati. 
• They also have MS and Ph.D. programs which indicates that they are also very active with regard to research. 
• They are being mentored by Prof. UB Desai who is also the director of IIT Hyderabad and a pioneer in System and Network Research in India. 

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.