A 10-year-old girl and two boys, aged seven and five months, were found dead inside the Anderson St property by firefighters who responded to the blaze on July 19.
The children are not being named for cultural reasons.
Their mother, Margaret Dale Hawke, 36, was charged with three counts of murder and one count of criminal damage by fire.
Court records indicate she pleaded guilty to all of the charges during an appearance in the Stirling Gardens Magistrates Court on Wednesday.
Hawke was remanded in custody and could face life in prison when she is sentenced next year.
Hawke was hospitalised at Hedland Health Campus after the fire before being flown to Perth for further specialist treatment.
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The blaze sent shockwaves through the community of Port Hedland in WA’s Pilbara region.
Support services were provided in the town by government agencies.
In a statement earlier this year, the Hawke family said it was going through a “very traumatic grief process” and asked for their privacy to be respected.
“We are quietly paying tribute to our three little angels, and just supporting each other at this time,” they said.
“Please let us do this without adding to our grief.”
