Police said that the ambulance driver lost control of the vehicle after one of its tyres burst early in the morning.
A 23-year-old pregnant woman and two relatives accompanying her in an ambulance were killed after the vehicle crashed into a roadside tree at Alathur near Sankarapuram in Kallakurichi district in the early hours of Thursday.
The victims were identified as K. Jayalakshmi, 23, of Sorapattu, her mother-in-law Selvi and sister-in-law S. Ambika, 32, of Sorapattu.
The ambulance was bringing the nine-month pregnant woman from Pudupattu Primary Health Centre to the Kallakurichi Government General Hospital.
Police said that the ambulance driver lost control of the vehicle after one of its tyres burst around 4.45 a.m.
While Selvi and Ambika died on the spot, Jayalakshmi died on the way to the hospital. Further investigations are on.