Const. Fiona Thivierge of Peel Regional Police says a 60-year-old man, his 59-year-old wife and their 36-year-old son died in hospital after the overnight incident in Brampton, Ont.
Two other men who were also in the home were taken to hospital but are expected to recover from inhalation of fumes.
Thivierge says the couple's 29-year-old son arrived home just before 2 a.m. Monday to find the carbon monoxide detector going off and everyone inside the home unconscious.
She says the home's furnace had broken down the previous day and at least one outside heater had been brought inside to warm the home during the bitterly cold weekend.
"All we really know at this point is the furnace... broke down at some point yesterday. They had these propane heaters at the side of the house and they went and brought them inside to warm the house up," said Thivierge.
It's not known if one or two heaters were brought inside or how long they were being used in the house, she said, describing the heaters as "salamander" units of the type used at construction sites.
The identities of the victims are not being released.
Thivierge says the coroner is investigating and police are assisting but the case is not considered a criminal one.
Police are reminding people not to bring propane heaters into their homes.
"All propane heaters are supposed to be used outside and properly ventilated," said Thivierge. "They're not supposed to be inside at all."
After December's ice storm left hundreds of thousands without power in Toronto and surrounding areas, officials warned of the deadly risk of carbon monoxide poisoning if outdoor equipment was brought indoors.
Two people died of carbon monoxide poisoning in Newcastle, Ont., east of Toronto, after trying to keep warm with a gas generator in a garage. In another incident, four Toronto residents were treated in hospital after using a charcoal barbecue indoors.