People at the home were using propane heaters in the house after a furnace broke down, police said.
One person came home just before 2 a.m. ET and found carbon monoxide alarms were sounding. Peel paramedics responded to the call and treated six people at the scene.
Five people were taken to hospital, where three were pronounced dead. Two people are being treated, their condition is not known.
The home is located on Linden Crescent in Brampton, in the Dixie Road and Queen Street East area.
Brampton is located about 45 kilometres northwest of Toronto.