The identities of the victims were not made public by Quebec provincial police, who said Tuesday that autopsies were needed to officially identify the remains and determine the cause of death.
But relatives and friends left notes of sympathy on the Facebook page of Jocelyn Marcoux, 47. His son, Lindsay Brillant-Marcoux, was 13, and his daughter, Karen Brillant-Marcoux, 11.
"Your children will always be with you now," one woman wrote. "Your gesture is not excusable, but is understandable. Be at peace with your angels."
One friend said: "I'll always remember you, Jocelyn. Look over us and rest in peace."
It appears a custody battle was at the heart of the tragedy.
In the hours leading up to the violent blaze, Marcoux published a lengthy diatribe on his Facebook page denouncing a court decision allowing his ex-wife to have partial custody of the children.
Marcoux noted he had full custody of the children, and one neighbour said the kids had been there a little more than a year.
However, the lawyer representing the ex-wife in the case said no decision had been made in the case. He said the couple were to appear in Quebec Superior Court on Tuesday to discuss custody and alimony issues.
In his letter, typed in French, Marcoux said fathers must take justice into their own hands because they can't get it from the court. He feared he would lose custody of the children.
"For all the fathers out there, it's official," the letter reads. "If you don't get justice yourself, you'll never have it."
The letter was posted early Tuesday on Facebook, not long before police received a call at 3 a.m. for a house fire in Warwick, about two hours of northeast of Montreal.
A detached garage beside the home was engulfed in flames when emergency personnel arrived, said Sgt. Ronald McInnis, a provincial police spokesman.
Traces of accelerants were found in the garage and the doors to the building were blocked by a car parked in front.
"After the firemen finished their work they found three bodies inside (the garage)," he said.
Richard Gagne, another provincial police spokesman, said autopsies could take several days.
He said he could not go into details of the investigation.
— Written by Sidhartha Banerjee in Montreal.