While all three institutes are prestigious institutes, my sister chose ISI Kolkata for a number of reasons. Here are a few reasons
ISI Kolkata is a great option as it offers opportunities in both, jobs and higher education.
ISI Kolkata is the best option for MSc in Maths, followed by IISc. IIT KGP is reputed in the technology domain, but for core science subjects, it’s not a good option. If you are interested to pursue pure Mathematics, choosing ISI is the best option among these. It specializes in the field. You will get better career opportunities and exposure in the field.
It would be better to focus on colleges that offer Mathematics as their core and you must keep technological institutions aside. Thus, IIT Kharagpur should not be considered.
Talking about IISc and ISI Kolkata, both are research-oriented institutions and are equally good for taking up MSc in mathematics. But when we talk Maths in specific, then it would be better if you look at ISI Kolkata as your ultimate option.
The institute is specifically oriented towards studies related to Mathematics. You will be given great exposure at ISI Kolkata.