Margaret Dale Hawke, 36, formally pleaded guilty at the Supreme Court in Perth on Friday to murdering her 10-year-old daughter and two sons, aged seven and four months.
They died in July last year in the family's Port Hedland home, sending shockwaves through the town of about 16,000 people.
The court heard Hawke strangled her daughter with an electrical cord and stabbed her eight times in her chest and heart.
Her seven-year-old son was found with three stab wounds to his chest and wounds on his neck from also being strangled.
Hawke admitted smothering her infant son before setting her home alight and walking into the street as it was engulfed by flames.
Prosecutor Justin Whalley SC told the court one witness described her as "really calm" as she watched it burn.
Another heard her say: "He's taken everything from me".
Mr Whalley said Hawke then screamed and cried, saying: "My babies, my babies ... You don't have to suffer anymore".
Members of the public tried to enter the home and save the children but it was too dangerous.
Hawke told police she didn't know of any other way to help her children before admitting to lighting the fire.
"My three babies ... I hurt my babies. I did it to my babies," she said.
Firefighters put out the blaze. Hawke's elder son's body was found on a mattress in a room at the front of the property. Her daughter and other son were found in a room at the back of it.
Hawke was taken to hospital where she later admitted to staff she had murdered her children.
"I don't know why I did what I did. Maybe to stop the pain in all of us," she told police in an interview after the incident, the court heard.
Mr Whalley said Hawke told the children she loved them and asked for their forgiveness before murdering her daughter, followed by her sons.
He said an arson investigation found the fire was deliberately lit with two ignition points in the house.
None of the children were found with soot in their respiratory systems indicating they likely died before the fire was lit.
In court on Friday, Hawke, who also pleaded guilty to wilful and lawful damage by fire, was overcome with grief.
She sobbed in the dock and repeatedly banged her head on a balustrade until security staff stopped her.
Judge Michael Lundberg listened to sentencing submissions before adjourning his sentencing decision until May 5.
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