How did artists like Sheba Chachi, Ravi Agarwal, and Atul Bhalla use photography to document societal changes?

By focusing on marginalized groups

Answer:  By focusing on marginalized groups
These artists photographed those who lived on the margins of society, such as women ascetics and queer people.

Photography could also be used to document changes in society as artists saw them. Sheba Chachi, Ravi Agarwal and Atul Bhalla, among others, photographed those, who lived on the margins of our society, whom we do not notice much in our day-to-day life — women ascetics, queer people, and so on. Often, they would express their concern about ecology like pollution of rivers and urban congestion. Photography and video have inspired many contemporary artists.