The case against Tania Clarence, 42, who has admitted the manslaughter of Olivia, four, and three-year-old twins Ben and Max by diminished responsibility will be outlined at the Old Bailey in London.
Accepting her plea last month, prosecutor Zoe Johnson QC said the mother-of-four killed the children “because she wanted to end their suffering” and at the time “she could not see any alternative or any other way out of their joint suffering”.
Justice Nigel Sweeney will hear the facts of the case and mitigation, but will sentence at a later date.
The court has heard that Clarence confessed to killing the children at her home in the wealthy south-west London suburb of New Malden on April 22.
She left three notes before allegedly smothering the children, who all had type two spinal muscular atrophy (SMA), a condition which affects muscle strength.
She then tried to kill herself by taking a drug overdose and slashing her wrists.
Clarence was discovered bleeding and crying by her nanny and a neighbour, who had gone to the home after becoming concerned for her safety.
They arrived at 9pm to find the home in darkness and put on a light on a mobile phone before going upstairs, calling out to Clarence as they went.
Ms Johnson said they “got to the first floor and found Mrs Clarence in a bedroom”.
“She was telling them to go away, saying that it was too late, and clearly seemed disturbed and devastated.”
Asked if she had taken anything, the court heard that Clarence replied: “I took something yesterday, but it didn’t work.”
Ms Johnson said that even from the light from the phone it was obvious she had tried to commit suicide as there was blood coming from her wrist.
Police were called and the neighbour checked if anyone else was in the house.
He opened the door into the children’s bedroom and saw the twin boys dead, but was too shocked to continue his search of the house.
The nanny told police that as she searched the house she was calling out to Clarence, but the South African shouted back: “Just go, it is too late, just go, just go.”
Police arrived at around 9.30pm and the three children were pronounced dead at the scene at 9.41pm.
Clarence confessed to the triple killing to a female police officer at the scene, saying: “I’m sorry. I’m sorry.” Asked why she was sorry, the tearful mother said: “I killed them. I suffocated them.”
Clarence was taken to St George’s Hospital where she was treated for a self-inflicted one-inch cut on her left wrist.
At the hospital she was told she was going to be taken to the police station, and cried out: “Why do I have to do this, I’m guilty,” the court heard.
Two other notes were found in the house. One discovered on top of a pile of clothes in the master bedroom read: “Gary, I don’t want to be saved please. I can’t live with the horror of what I have done. I thought the pills would work, they didn’t.”
A third note was addressed to the nanny, who was described in court as “a significant member of the family”. Its contents were not divulged.
The court heard that Clarence’s husband Gary was away in South Africa and was not due to fly home until April 24.
Clarence was not at the Old Bailey to enter her plea in person, although her husband, an investment banker, has attended every hearing in the case.
Children with SMA type two have weakness in their muscles which means they cannot stand unaided, may have difficulty with other movements and are vulnerable to respiratory infections.
Although it remains a life-limiting condition, improvements in care mean that most of those affected can live long and fulfilling lives.