San Joaquin Deputy District Attorney Tori Verber Salazar said further charges would be brought against Wesley Brownlee, who is accused of homicides in Stockton, California.
Brownlee may face life without parole or the death sentence if convicted.
In addition to the three murder charges announced Tuesday, police are investigating two killings, an attempted murder, and another murder in Oakland, according to Salazar.
Ballistics evidence links the seven incidents, but investigations are continuing.
Prosecutors said the pistol recovered in the suspect's possession was a "ghost gun" used in all six killings.
Ghost guns are untraceable, self-assembled weapons generally made in 30 minutes from internet components.
In the time of his arrest, "Mr. Brownlee was spotted stalking unsuspecting victims at the nearby municipal park," the prosecution claimed in court.