The taxi driver and his two friends were arrested later.
Drink-driving wiped out an entire family in Chennai on Monday. The victims - a man and two women - were ragpickers sleeping on a pavement in the city's Velacherry area, an IT hub.
The vehicle was a taxi contracted by a local call centre. The driver, off duty, had been out on a joy ride with two friends. All three were drunk.
The men were arrested by Monday evening. Investigators said it was a clear case of drink-driving and over-speeding.
The driver had told police that he had lost control of the car when he swerved to avoid hitting a cow. "But the car hit the cow, which died," said a police officer. "And it halted only after running over the three people sleeping on the pavement 30 metres away."
One of the two women, Aishwarya, a 28-year-old, was pregnant. Her husband had died too. Their child, a one year-old, was saved since he was sleeping on a cart. Police are yet to establish the identity of the other woman.
Hussein, a pavement dweller, who was nearby and had a close shave, said, "The car was out of control and over-speeding. We all lived like a family helping each other. The child is now an orphan".
Tamil Nadu has a poor road safety record. Over the last two years, nearly 32,000 people died in road accidents across the state. Around 1900 of them - six percent - were victims of drink-driving.