The woman who was driving the car was in front when a man, a woman and two children came from behind and all four were said to be stuck in the window.
The three-wheeler had veered off the road and overturned on a paddy field.
The woman and her two children who were seriously injured in the accident were admitted to the Wathupitiwala Hospital and later succumbed to their injuries.
The deceased has been identified as a 40-year-old resident of Kirindiwela Road, Veyangoda. Both children who died were sons. One son is nine years old, and the other son is one and a half years old.
The other woman has been transferred to the Colombo National Hospital for further treatment.
Below is CCTV footage of the crash.