Her shock guilty pleas stunned the courtroom.
She appeared alongside her boyfriend Gary Richards, who denied the two attempted murders and three charges of preventing burial.
Richards, who is 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".
Two other defendants, Leslie Layton and Robert Moore, appeared at the hearing by videolink.
Layton, 36, pleaded not guilty to perverting the course of justice at an earlier hearing, while Moore, 55, denied assisting an offender.
- AFP
-AAP