Jason Karels pleaded guilty, but mentally ill, in the murders, according to the Lake County (IL) State's Attorney's Office.
Prosecutors said he drowned the children - ages 2, 3 and 5 - in June of 2022. Their mother found the bodies at Karel's Round Lake Beach home at 200 East Camden Lane
Officers said he wrote a note reading, "If I can't have them, neither can you."
Karels has previously pleaded not guilty to the murders.
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Karels will be sentenced to life in prison, without the possibility of parole, authorities said.