As police units with dogs were called in, Scotland Yard warned the public not to approach the woman charged with murder, urging anyone who sees her to dial 999.
Melanie Myers, 19, of Neasden, north-west London, is charged with murdering Nazhar Hussain in Cricklewood on 12 April. She was described as of mixed-race appearance, 5ft 10in tall, of medium build, with black shoulder-length hair in a pony tail. She was wearing a blue jumper and possibly black leggings.
The second escaper is Jean Jarvis, 26, of Bow, east London, white, 5ft 7in and slim, with tattoos of a swallow and a heart on her right arm and of the words 'Motown' and 'Soul' on her left shoulder. She is charged with burglary.
The third, Stephanie Osbourne, 26, of East Finchley, north London, is charged with supplying drugs. She was described as white, 5ft 5in, of medium build, with 'I love Martin' tattooed on her right hand.
A prison service spokeswoman said that at about 3.10pm the three remand prisoners had slipped away from a group of 32 women being escorted back to their cells from the exercise yard.
They had apparently broken a window to reach the roof of the prison swimming pool. That gave access to the perimeter wall.
Scotland Yard said witnesses had seen the three women near the prison. They had used a telephone box for a short call, and then walked away north along Camden Road.
Derek Lewis, director general of the prison service, has asked the prison's governor, Janet King, for an urgent report, and will launch an inquiry today.