Police are looking for a fifth employee of the security company, Edmonton Police Chief Rod Knecht said. The armored truck, abandoned but running, was found not far from the security company's offices.
The police chief said Travis Baumgartner, a 22-year-old employee of G4S Cash Services, is a person of interest and his involvement in the shootings is not clear. Knecht said authorities in the U.S. have been notified as well.
Police said no University of Alberta students were involved. The university was quickly put on lockdown, but that was later lifted.
G4S Cash Services spokeswoman Robin Steinberg confirmed that two male employees and a female employee were among the dead. She said the guards were armed, but she did not have further details.
Steinberg said the company has never had a fatality in Canada before.
"It's horrible to lose this many," she said.
One G4S Cash Services van was found at the scene. Police found an armored truck, running but abandoned, not far from the G4S offices.
"At some point during the delivery, three of those employees were shot at a location inside the mall," the police chief said during a press conference. "A fourth employee was shot outside the mall."
Such shootings are rare in Canada, where residents are nervous about anything that might indicate they are moving closer to U.S. levels of gun violence. The oil boom town of Fort McMurray, Alberta, however, often has drug-related shootings, and Vancouver often has gang shootings. Gunfire at Toronto's most prominent mall this month left two dead and several injured. A shooting at an Edmonton club in 2006 left three dead.
It was the second robbery of a G4S armored vehicle in Edmonton in recent months. Last December, guards making a mid-afternoon pickup outside a casino were attacked and pepper-sprayed by two masked men. The pair fled in a Jeep with an undisclosed amount of money.
Friday's shooting happened shortly after midnight in the Hub Mall area, which has student residences and shops. It's the university's summer session, so fewer students are living on campus.
Ian Breitzke, a student, said he was watching TV when he heard a man crying out in pain. He said when police came about 10 minutes later they broke down the door to a room behind an ATM machine and pulled out two people who seemed dead.
The student said police then pulled out a man who was alive.
One photo posted to Facebook was taken from a balcony looking down into the mall. It shows three people lying in front of a TD bank machine, with emergency crews working over them. Blood streaks the floor from behind the machine to where the bodies are.
"The university is saddened about those who lost their lives last night and we extend our condolences to their loved ones," the university said on its website.
It said the rest of the campus was operating as normal.
Baumgartner was believed to be driving a Ford F-150 truck with Alberta license plate ZRE-724, authorities said.