The ambulance was on its way to the Trivandrum Medical college hospital when the mishap occurred at Kunnikode near Punalur this morning.
Three persons-- the 65-year-old woman, her 33-year-old grandson and the ambulance driver died on the spot-- while the woman's 45-year-old daughter, who was seriously injured, succumbed at the medical college hospital, police said.
Another woman, a relative, who was also injured, is undergoing treatment, police said.
The bus was on its way to Punalur from Kollam.