IIT Kanpur is a better option to study physics as compared to St. Stephens or IISc. Being a student of IIT Kanpur myself I can vouch for the institution.
Usually, the choice of the Institute depends on what you want from your degree and your career prospects. However, knowing a bit about each Institute will help you decide better.
St. Stephen's: This Institute gives minimal or no emphasis to research work. The curriculum is purely theoretical so aren’t taught the concepts in-depth. It gives you a general view of concepts and teaches you much more than other colleges. The teachers are highly qualified but aren’t involved in research work. So we can say, this Institute is the perfect choice for those who love physics, want to study it, and get a degree but don’t want to pursue a career in the same. The excellent faculty combined with top-quality education renders you employable in the Indian job market.
IIT Kanpur: Physics department is well known for its excellent placements, cutting-edge research work, and tremendous higher education opportunities. The IIT tag gives you recognition in US universities where you can go for higher education. Even if you don’t want to pursue a career in physics, the program inculcates excellent skills that help you land a decent job.
IISc: This is a premiere research institute and heaven for research enthusiasts. The campus, facilities and overall environment is conducive to conducting top-quality research. It’s well known for its interdisciplinary research which is even better than IIT Kanpur. Moreover, it enjoys the local advantage of Bangalore, the Mecca of Indian science. Collaborating with scientists from RRI, JNCASR, NCBS, IIA, etc. is also possible at this institute.
If you don't want to pursue Physics after your Bachelor's degree, St.Stephen's is best for you. But If you are not sure of your career interests then IIT Kanpur is a good option lastly, if you are a hardcore physics lover and want to pursue a career in the same IISC is the pl