An Illinois dad who wrote his estranged wife that “if I can’t have them, neither can you” before murdering their three young children has pleaded guilty to the slayings — though he insists he was suffering from mental illness.

Jason Karels, 36, of Round Lake Beach, will be sentenced to life behind bars without the possibility of parole, Lake County State’s Attorney Eric Rinehart said Friday after the defendant pleaded guilty but mentally ill to three counts of first-degree murder.

Karels admitted drowning 5-year-old Bryant Karels, 3-year-old Cassidy Karels and 2-year-old Gideon Karels in a bathtub on June 13, 2022.

Their mother, Debra, found the bodies at Karels’ home at 200 E. Camden Lane in the Chicago suburb, where police said he left a note reading, “If I can’t have them, neither can you.”

“This case has devastated the Round Lake Beach community and beyond,” Rinehart said in a statement. “We hope that today’s resolution provides some measure of closure, and we will continue to provide support to the family.”

Several members of the family attended the court hearing on Friday.

Karels was arrested after leading police on a chase that ended in a crash at an Interstate 80 bridge in Joliet, police have said. He was briefly hospitalized after the crash.

After the crash, he told police he was responsible for his kids’ deaths in a bathtub and that he had tried to kill himself before fleeing the home, where officers found his blood from the suicide attempt, police said.

The parents shared custody of the children but did not live together at the time of the deaths, officials said.

A relative has claimed that Jason suffers from mental illness but had been refusing to seek psychiatric help, and Debra was in the process of getting a divorce after allegedly suffering abuse at his hands.

But the mom wanted to maintain a relationship with Jason for the sake of their children, and allowed them to stay with him.

Karels will be sentenced to life in prison on Feb. 16.

With Post wires