Authorities called to a home on Turcotte Street around 4 p.m. ET found the bodies of a four-year-old boy and his two sisters, five and two years old.
Police said the children's deaths have been deemed suspicious.
Sgt. Christine Coulombe, spokeswoman for Quebec provincial police, said the children's mother was taken to hospital for reasons police have yet to determine.
The 33-year-old mother is "considered an important witness to the event and will be met by investigators once her health permits," Coulombe said.
Neighbours who spoke to CBC News said the mother had lived alone with the three children in the second floor of a duplex for about two years.
According to neighbours, the children lived there part-time.
Drummondville is about 90 kilometres northeast of Montreal.