Jenny Hayes sat in her car outside a townhouse for four minutes, where she texted a man who had wronged her "I'm setting your house on fire right now".
She then stormed into the southwest Melbourne home and lit a deadly fire, killing a young couple and their 19-day-old baby.
These actions demonstrated a level of premeditation, despite Hayes not knowing the family was asleep upstairs when she started the blaze, prosecutors have told the Supreme Court in Melbourne.
"The evidence is there was a degree of forethought before she embarked on this destructive exercise," crown barrister Raymond Gibson KC said on Tuesday.
"She communicated with the person who had wronged her that he was going to jail. Within minutes or seconds she's in the premises, goes into the open doors of the downstairs bedroom and then she uses a lighter to set fire to the premises."
Then, instead of calling emergency services, she took photos of the townhouse engulfed in flames and texted someone before leaving in her car, he said.
Hayes looked down and remained silent as she faced the second day of a pre-sentence hearing by video link from prison.
Mr Gibson called for Hayes to be handed the maximum penalty of 25 years in prison, after she pleaded guilty to three charges of arson causing death.
Abbey Forrest, 19, her partner Inderpal Sohal, 28, and their three-week-old daughter Ivy died from carbon monoxide poisoning in December 2020 fire.
They were all sleeping in an upstairs bedroom when their Point Cook home was set alight, in the early hours of December 2.
Mr Sohal had allowed one of his friends to use the downstairs room, the court heard previously. The friend hired Hayes, a sex worker, and she provided services which were paid for but there was a dispute and the man left.
Hayes claimed he raped her and stole cash from her, then sent him a series of angry texts, including telling him she would set fire to his house.
She took photos as the townhouse was engulfed by flames after setting fire to the man's mattress.
"I'm so sick of people thinking they can do this s*** to me and nothing happens to them," she said as she left, in a text message.
Her barrister Theo Kassimatis KC said, while the fire was an "unspeakable tragedy", Hayes should receive a low non-parole period due to her traumatic background, early offers to plead guilty, depression and personality disorder.
He said she continued to suffer shame and express remorse over her offending, as she did not know the young couple and their baby were upstairs.
"This is a woman who, when she was told three people had perished, she was hysterical," Mr Kassimatis said.
"She was screaming into the phone that she had killed a baby."
Justice Elizabeth Hollingworth will hand down Hayes' sentence on Friday.