Rendell's office said Thursday the governor signed the commutations for Keith O. Smith, William Fultz and Tyrone Werts.
All three have served decades in state prison after being convicted for their roles in separate Philadelphia slayings in the 1970s.
The governor's decision follows unanimous recommendations from the Pardons Board earlier this month in favor of commutation.
Rendell's spokesman says he intends to release a statement later Thursday.
Rendell spokesman GaryTuma said on Wednesday that Rendell's decisions were influenced by several factors: the men's role as accomplices rather than triggermen in the slayings, their incarceration of many years, the lighter sentences for others involved in the killings, and the unanimous clemency recommendation from the state Pardons Board.
Tuma said Rendell will impose conditions that if the men are convicted of a criminal offense or violate parole, the commutation may be revoked by the governor upon the recommendation of the Pardons Board and after a public hearing.
William DiMascio, executive director of the Pennsylvania Prison Society prisoner advocacy group, knows all three and described them as good candidates for release.
"I think every one of them served 35 years," DiMascio said. "That was a lot of time for them to serve, considering their involvement in the crime. It's not to make light of what happened or in any way to demean the grievousness of what took place, but I think it's time for the state to show mercy."
The 4-0 votes by the Pardons Board earlier this month - Attorney General Tom Corbett, the governor-elect, was absent - were the first such votes in more than six years.
An October ruling by the 3rd U.S. Circuit Court of Appeals, upholding a 1997 constitutional amendment requiring unanimous votes for lifers or people on death row, cleared the way for the Pardons Board to consider the three applications.
Smith, 55, a Chester State Prison inmate, was arrested in 1974 and convicted of second-degree murder in the shooting death of a store clerk, according to the 2006 clemency application he submitted to the board.
Fultz, 58, an inmate at Graterford State Prison, was convicted of first-degree murder in the slaying of a teenager in 1974. His 2006 clemency application said that he discarded the shotgun afterward and that he "hung out with the wrong crowd" at the time.
Wertz, 60, also incarcerated at Graterford, described his crime in a 2004 application as a robbery conspiracy that ended in a killing as he sat in the car. He was convicted of second-degree murder and conspiracy, his application said.
"The fact of the matter was, I was as guilty as if I had pulled the trigger myself," Wertz wrote.
Those sentenced to life in Pennsylvania are not eligible for release unless they receive a commutation. There are nearly 5,000 lifers and more than 200 death row inmates in the state prison system.