Vanessa George, Angela Allen, from Nottingham, and Colin Blanchard, from Rochdale, all 39, had never met in person before they appeared in court.
Abuse was recorded on mobile phones and images exchanged via e-mail and text.
Police, who described the abuse as "horrific", are continuing to try to identify victims at George's nursery. The three will be sentenced next month.
Children's Secretary Ed Balls said it was a "deeply distressing and disturbing case" and he expected the serious case review to be completed as soon as possible.
"It is vital we find out how an adult could abuse their position of trust in such an evil way and do everything we can to prevent this kind of abuse happening in the future," he said.
George, Allen and Blanchard met on social networking website Facebook, although it is unclear when.
Detective Superintendent Adrian Pearson, of Nottinghamshire Police, said the trio had been guilty of child abuse in its most "horrific and devilish form".
Nottinghamshire Police spokesman: "The cunning they have all used for their own ends is really, really shocking"
"Those three individuals have shared quite willingly and freely images, texts, fantasies of the most serious level you could imagine," he said.
Ann Reddrop, from the Crown Prosecution Service, said their crimes were beyond comprehension.
"These three individuals each acted in a way which ordinary people will find hard to understand," she said.
"They showed total disregard for the lives of their victims, their own families and those they worked with, all of whom have been left devastated by these crimes."
The public gallery at Bristol Crown Court was packed on Thursday with about 30 parents who wept as the guilty pleas were entered.
One man pointed at Blanchard and was asked to calm himself by the usher. Allen wept as she pleaded guilty while Blanchard showed no reaction and George hung her head in the dock.
She's caused massive trauma to a great number of victims
Chilling bond between abusers
'I can't believe how evil she is'
Spotlight on abusers
The abuse began in September last year and the three were arrested in June following a police investigation involving officers from the forces in Devon and Cornwall, Nottinghamshire and Greater Manchester.
The trio pleaded guilty to 37 out of 38 counts.
George, who had worked at Little Ted's nursery in the Efford area of Plymouth for three years, admitted 13 charges, including sexual abuse of children and making and distributing indecent images of children.
She pleaded not guilty to one charge of sexual assault which was ordered by the judge to lie on file.
Her arrest sparked massive public outrage and angry scenes during earlier court appearances.
Allen, of Nottingham, admitted five counts of sexual assault and distributing indecent images of children.
IT worker Blanchard, of Smallbridge, near Rochdale, admitted 19 counts of sexual assault and distributing indecent images of children.
Jon Kay has been to Plymouth to meet one mother who trusted Vanessa George to look after her children.
They exchanged thousands of e-mail and text messages containing images of child abuse, the court heard.
Blanchard was arrested on 6 June after one of his colleagues found obscene pictures on a work computer.
Examination of his laptop computer led to the arrests of George and Allen.
No-one else is believed to have been involved.
Judge John Royce, who adjourned sentencing for reports, warned the three abusers they faced substantial prison sentences.
He said the "decent" thing for George to do would be to co-operate with police in identifying all the abuse victims in the photographs.
Speaking to George's lawyer, the judge said: "Your client must know it seems to me who she has abused and who she has not. If I were a parent, I would want to know whether my child was abused or not."
Det Supt Michele Slevin, from Devon and Cornwall Police, said experts had been unable to identify any of the children filmed by George.
She said George had been interviewed five times and had not given any name, but identifying the children remained their "priority".
"It's clear she's caused massive trauma to a great number of victims, not just the children involved but the families and community within Plymouth," she said.
Detectives have warned there is a high probability the children will never be identified.
It is not known when the three first met but Detective Inspector Tony Creely, of Greater Manchester Police's sexual crime unit, said they "were as bad as each other".
"Blanchard, George and Allen sent thousands of messages between each of them. Blanchard expressed love for each of the women, and they would reciprocate," he said.
"They would discuss sexual matters of a crude and gross nature and the abuse of children in the texts and e-mails.
"All of them were getting gratification from the texts and seeing who could have the worst idea."
http://news.bbc.co.uk/1/hi/england/8284192.stm
This story is awful, made me cry at work
So we can't even send our children to nursery without thinking about horrific things like this?
