Emergency services were called to the brick home in Sydenham, in Melbourne's north-western suburbs, about 9.40pm on Sunday to find smoke and flames billowing from the roof.
It took fire crews up to 30 minutes to retrieve the siblings, aged five, three and one, before they were rushed to the Royal Children's Hospital where they all remain in a critical condition on Tuesday.
Firefighters found three children, aged five, three and one, unconscious in the home. (Con Chronis/AAP PHOTOS)
The father of the middle child said the three were "happy little kids" and must have felt "petrified" during the ordeal.
"I hope they didn't feel anything to be honest," he told news crews outside the hospital on Tuesday.
"Obviously they would have, but I just tell myself they didn't.
"It shouldn't have happened."
The three and one-year-old children did not suffer physical burns but had to be treated for severe smoke inhalation while the man said the eldest child, whom he step-parented, may not survive.
Detectives are investigating the circumstances of the blaze and trying to establish if any adults were home at the time.
Investigators believe the fire started inside a room at the rear of the property with Detective Acting Inspector Adam Henry ending speculation that fireworks were involved, despite neighbours hearing a loud bang before the fire.
Detectives believe the blaze began in the rear of the property. (Con Chronis/AAP PHOTOS)
Det Henry confirmed police had spoken briefly to the children's distressed mother, who was at the hospital.
He was unable to clarify if she, or someone else, was at home with her children at the time of the fire and said it was too early to say whether the fire was suspicious.
The mother and children had only moved into the property a week earlier, he said.
It took 30 firefighters and 10 appliances, including an aerial unit, several hours to extinguish the blaze.
Firefighters located an apparently uninjured pet dog at the scene on Monday morning.
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