Hello,
You can cover chennai, Mahabalipuram and Pondichery in 3 days.
Tamilnadu public transport is the cheapest in India.
Mahabalipuram is like 1 hour drive in public transport from chennai. Rest all is OK in chennai Except the whether, it is very humid there. I was lived in chennai for 3 years for my studies 10 years back.