The trio's bodies were found in the "horrific" remains of the blaze after it engulfed a house at Gladstone, in central Queensland, on Wednesday, causing it to partially collapse.
The victims were reportedly two girls and a woman.
"Obviously it is a tragic event," acting Detective Acting Superintendent Luke Peachey told reporters.
"At this stage we have not established the identity of those three persons but we hope to do that as soon as we can."
Emergency services were called to the Toolooa home at 5.56am following reports of a fire, with multiple units attending.
"Upon arrival, the house was fully engulfed," police said in a statement.
Multiple occupants of the house were unaccounted for as emergency services battled the blaze.
The fire was extinguished about an hour later and a crime scene declared.
Search efforts were initially hampered after the top storey collapsed, before emergency services made the grisly discovery of three bodies.
Police initially did not rule out finding more victims due to uncertainty over the number of people who resided at the home.
"It's sort of a little bit iffy at the moment," Det Supt Peachey said.
"It seems to be a house that people come and go from.
"So until we're able to establish exactly how many people were in that, I won't speculate."
Neighbours called emergency services about the blaze after they were reportedly woken up by explosions and what sounded like a thunderstorm.
It may take days to determine the full circumstances of the fire, police said.
"At this stage we are treating it as unknown," Det Supt Peachey said of the fire's cause.
"Obviously the fire scene is very horrific … and will take some time to go through."
The victims' families would be supported as authorities tried to piece together what caused the fatal blaze, police said.
"We'd like to pass on our sincere condolences to all those families who are affected," Det Supt Peachey said.
"We'll be working with the families, once we establish who those people are, to try and work out what happened."
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