Montgomery County police are seeking the suspected gunman, who they named as 35-year-old Bradley William Stone.
They say he may be armed and dangerous, telling residents to remain indoors.
The victims are all said to have a familial relationship to Bradley William Stone, who has been described as having red hair and wearing military fatigues.
He also reportedly uses a cane or a walking stick.
The shootings in Harleysville, Lansdale and Souderton began at about 04:00 local time, according to local media reports.
One of the victims is believed to be a woman who was killed in her house.
Neighbors said they saw the suspect leave the house with two children, amid reports that the shooting may have been triggered by a custody dispute.
The children were reported to have been found safe a short distance away.
Police officers later surrounded a home in Pennsburg, where the suspect is a resident.
Shelter in place orders were issued for local schools.
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