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.
The children's father, who is estranged from the mother, told reporters he was supposed to visit with the children on Tuesday. Instead, he left flowers and mementoes at the scene of their death.
``Today I lost parts of me,'' said Paul Laskaris.
``I don't know really how to deal with it. I went to the hospital, I'm under medication. It's very difficult.''
Television news footage showed the children's mother running up to the charred home before she was restrained by emergency officials. The woman screamed as she was led away. She was treated for shock.
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.
Municipal official Benoit Richard said an investigation will determine the cause of the fire, noting the blaze was confined to one apartment.
Meanwhile, Laskaris said he'll try to piece together his shattered life.
``I had the two most beautiful kids that you can imagine.''
