The couple were found hanging from the ceiling fan while the boy was hanging from the bar of a window, they said.
Police identified the victims as Thampan (56), his wife Padmini (45) and son Karthik (12), who was said to be suffering from serious ill-effects of endosulfan pesticide spraying in cashew plantation areas in north Kerala.
Thampan's two children had earlier died due to ill-effects of the pesticide.
The family had come to the hospital for treatment of their son yesterday.