To the shock of her lawyer, 30-year-old Joanna Dennehy admitted she killed the men, whose bodies were found in March and April riddled with stab wounds.
Her barrister Nigel Lickley told London's Old Bailey court,"The course of the arraignment is not one we had anticipated."
Dennehy, who has a star tattooed under her eye, told the judge, "I've pleaded guilty, and that's that."
The killer, who is from the city of Peterborough in eastern England, also admitted attempting to murder another two men.
Her shock guilty pleas stunned the courtroom.
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Richards, who is seven feet three inches (2.21 metres) tall, also goes by the name of Gary Stretch.
Dennehy further admitted preventing the lawful and decent burial of the three murder victims.
The bodies of Kevin Lee, 48, Lukasz Slaboszewski, 31, and John Chapman, 56, were found in ditches on the outskirts of Peterborough.
Police had launched a nationwide manhunt for Dennehy, warning the public not to approach her and describing her as having "a very distinctive green tattoo on her right cheek, beneath her eye".
Layton (36) pleaded not guilty to perverting the course of justice at an earlier hearing, while Moore (55) denied assisting an offender.