The bodies of Lukasz Slaboszewski, 31, Kevin Lee, 48, and John Chapman, 56, were found in ditches 10 miles (16km) apart in Newborough and Thorney.
Joanna Dennehy, 31, of Bifield, Orton Goldhay, Peterborough, pleaded guilty at the Old Bailey earlier.
The bodies of the victims were found in March and April. All three had been stabbed.
Dennehy admitting killing Mr Slaboszewski between 19 and 29 March. He had been stabbed in the heart, post-mortem tests showed.
She also pleaded guilty to murdering both Kevin Lee and John Chapman on 29 March.
Mr Lee died from stab wounds to the chest, and Mr Chapman had been stabbed in the neck and chest, tests revealed.
Dennehy also admitted "preventing the lawful and decent burial" of all three murder victims.
She also pleaded guilty to two additional charges of attempting to murder two other men.
After entering her pleas, Dennehy told the judge, Mr Justice Sweeney: "I've pleaded guilty, and that's that."
Her barrister, Nigel Lickley QC, said: "The course of the arraignment is not one we had anticipated."
Also in the dock was Gary Richards, 47, of Riseholme, Orton Goldhay, Peterborough - also known as Gary Stretch - who denied three charges of preventing the lawful burial of the bodies of Mr Slaboszewski, Mr Lee and Mr Chapman.
He also denied attempting to murder the two other men.
Two other defendants appeared at the hearing by videolink.
Leslie Layton, 36, of Bifield, Orton Goldhay, pleaded not guilty to perverting the course of justice at an earlier hearing, while Robert Moore, 55, of Belvoir Way, Peterborough, denied assisting an offender.
A woman has pleaded guilty to murdering three men whose bodies were found in ditches in Cambridgeshire.