Richard Poplawski, 24, is charged with three counts of homicide in the April 2009 shootings at his Stanton Heights home.
Officers Paul J. Sciullo II, Stephen J. Mayhle and Eric G. Kelly were responding to a 911 call from Poplawski's mother, who reported a domestic dispute with her son, the Pittsburgh Post-Gazette reported Monday.
Security was tight in the courtroom of Common Pleas Judge Jeffrey A. Manning and a metal detector had been placed outside the courtroom, the Pittsburgh Tribune-Review reported.
About 150 police officers lined the street outside the Allegheny County Courthouse as opening arguments were set to begin, the newspaper said.
Because of extensive pretrial publicity in Allegheny County, 12 jurors and six alternates from Dauphin Country are hearing the case.
Poplawski also is charged with nine counts of attempted homicide for firing shots at other officers and wounding one of them during a 3-hour standoff.
The prosecution says it is seeking the death penalty.