Three persons from Pollachi were killed when the car in which they were travelling rammed into a bus at Virudhunagar early on Tuesday.
The police identified the deceased as Subramaniam, 78, Kamalam, 72, and their relative Sekar, 52.
Subramaniam's son Ayyappan, 56, who was behind the wheel, has been admitted to the Government Rajaji Hospital in Madurai with serious injuries.
The police said that they were proceeding towards Thiruvananthapuram to visit a patient admitted to a hospital.
When the car was entering into Virudhunagar town around 4.45 a.m., it rammed into the rear of an SETC bus which was moving out of the carriageway of the Madurai-Tirunelveli four-way highway in front of the TNSTC depot.
The bus, with sleeper facility, was proceeding towards Nagerkoil from Bengaluru.
Police and Fire and Rescue Service personnel had a tough time retrieving the car which got trapped under the bus.
Bus driver N. Vijayan, 52, has been picked up for enquiry.
