The incident occurred at Manamai near Mamallapuram when the trio (a man, his wife and daughter) were returning to Chennai after visiting the place to take a look at a residential layout, police said.
The car's engine was idling for some time when it caught fire, they added.
The locals, who noticed the car catching fire alerted personnel of the Fire department and police.
Even before the blaze was fully doused by Fire and Rescue Services personnel, the car was gutted and the three passengers were charred to death.
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