A depressed mother wept in the dock as a court heard how she smothered her disabled twin sons with a nappy before killing their sister and trying to kill herself.
Tania Clarence (43) has admitted the manslaughter by diminished responsibility of Olivia (4) and three-year-olds Ben and Max at her home in New Malden, London, over the Easter holidays.
The children were found dead - tucked into their beds with toys arranged around their heads.
Prosecutor Zoe Johnson QC told the Old Bailey the children suffered from the muscle-weakening condition SMA type 2.
Days before the killings, Clarence's husband had taken their eight-year-old daughter, who is not disabled, on a trip to South Africa, leaving the defendant alone.
Ms Johnson said: "She smothered the boys first whilst they were sleeping using a nappy so they would not smell her.
"She found it much harder to kill Olivia, and wrote a letter to her husband in the time between killing the boys and killing Olivia."
The deaths were discovered after a nanny, called Jade, was phoned by Clarence's mother.