At a news conference on Tuesday, Weld County District Attorney Michael Rourke called Watts' story "a flat-out lie" and said he deserved a life sentence for each of the nine counts to which he had pleaded guilty.
Shanann Watts, center, and her children are seen in this photo posted to Facebook. Her brother looked steadily at Watts throughout the hearing, which lasted about 30 minutes.
The victims' family agreed to those terms, the district attorney's office added.
Shanann, Bella and Celeste were reported missing August 13 from their home in Frederick, a town of about 8600 people about 50 kilometres north of Denver.
His wife was buried there and the girls were submerged in crude oil, with their remains found by police who used a drone to search areas identified by Watts.
Investigators discovered Watts was "actively involved in an affair with a co-worker", which he had denied during an earlier interview.
He also pleaded guilty to unlawful termination of a pregnancy, as Shanann Watts was 15 weeks pregnant with a son her family said would have been named Nico, and three counts of tampering with a deceased human body.
Watts acknowledged strangling his wife, Shanann, 34, and their daughters, Bella, 4, and Celeste, 3, and dumping their bodies in an oil field belonging to Anadarko Petroleum, where Watts had worked until he was arrested in the case, according to Denver 7 News.
Watts had faced the possibility of the death penalty if convicted, but under his plea agreement prosecutors pledged to not seek the death penalty, Steve Wrenn, chief deputy district attorney for Weld County, told the judge.
Rourke said he described "extraordinary delays" in carrying out the death penalty in Colorado since Gov. John Hickenlooper's 2013 decision to block the execution of Nathan Dunlap.
Rourke said he hoped the agreement would bring some measure of closure for the family, who were present during the court hearing and the press conference. "I don't want to be in the position to make that choice to take his, '" Rourke said.
"I don't know how I could have sat in Sandy (Rzucek)'s kitchen and asked her to commit the next 25 years of her life of the criminal justice system", Rourke said. Rourke said he was "sick" and "saddened' over the case". "The spotlight shines directly where it belongs: On him".
'No one wins today.
Watts is represented by the Office of the Colorado State Public Defender, which does not comment on pending cases.