The bodies of the children were found Saturday, hours after aand jailed on $5 million bond in the deaths of Jimella Tunstall, 23, who was seven months pregnant.
The children were found together, the St. Clair County coroner's office said. The coroner would not give the location.
Police began a search for Tunstall's two sons, 7 and 2, and 1-year-old daughter, since they found the mother's body last week in a weedy East St. Louis lot.
Authorities had continued a desperate search for Tunstall's children after charges against Tiffany Hall, 24, were announced Saturday. The children were last seen with Hall on Monday, three days before she was taken into custody.
Relatives of both women told media outlets the two grew up together and attended alternative schools. The woman in custody often baby-sat Tunstall's children, and Tunstall never expressed worry about leaving them in her care, the St. Louis Post-Dispatch reported.
An autopsy showed Tunstall bled to death after sustaining an abdominal wound caused by a sharp object, believed to be scissors, said Ace Hart, a deputy St. Clair County coroner. Investigators told CBS affiliate KMOV-TV in St. Louis that the baby was most likely cut from the victim's womb last week.
Hart said he believes Tunstall was knocked unconscious before her baby, seven months into gestation, was removed during a slaying he called "very graphic and very brutal."
Hall will likely be arraigned Monday on the charges, each carrying a 20 to 60 years or life in prison, said St. Clair County State's Attorney Robert Haida. The murder count could be punishable by the death penalty.
Officials suspect Tunstall was slain on or about Sept. 15, Haida said.
The same day, Hall summoned police to a park, saying she had gone into labor, Hart said. The dead baby, taken to a hospital, showed no signs of trauma, and an autopsy the next day failed to pinpoint a cause of death, he said.
Hall would not let doctors at the hospital examine her and offered conflicting reasons for why she went into labor, alternately saying she had consensual sex and was raped, Hart said.
Authorities say Hall acknowledged to her boyfriend during the baby's funeral Thursday that the child wasn't his, and that she killed the mother. The boyfriend told police, who arrested his girlfriend hours later, investigators said.
DNA tests should determine definitively whether the baby was the one Tunstall was carrying, Hart said.
The baby was buried Thursday as Taylor Horn after a funeral arranged by L. King Funeral Chapel, whose president said Hall called minutes after the service was to start, asking if she could reschedule for a different day so more relatives could attend. At the time, Levi King said, only two relatives were there.
The woman showed up two hours late, ultimately signing an affidavit for the funeral home stating that the child was hers, King said.
The East St. Louis case is the second recent case in the area involving babies.
Shannon Torrez, 36, of Lonedell, Mo. south of St. Louis, about an hour's drive from here is accused of slashing a young mother's throat and kidnapping her baby on Sept. 15. The baby was returned unharmed Tuesday, the same day Torrez was arrested.
Also in Missouri, Lisa Montgomery will stand trial April 30 on charges of snatching a baby from the womb of Bobbie Jo Stinnett at her Skidmore, Mo., home in 2004. The baby survived.