The fire was already burning hard by the time firefighters arrived at the scene in Joliette, about 50 kilometres from Montreal.
Police said the dead were a four-year-old boy, a two-year-old girl and a woman in her 50s.
A neighbour said he heard an explosion before the fire.
``I heard some loud booms,'' the man said. ``I thought it was a storm.''
He said he tried to rescue the occupants but the flames beat him back.
The blaze began in the basement but did not spread to the rest of the building.
The cause of the fire was not immediately known. Provincial police were investigating.
The children's mother arrived at the house and had to be treated for shock.
Municipal official Benoit Richard said an investigation will determine the cause of the fire, noting the blaze was confined to one apartment.
