Brown Deer police said they got a call of shots fired at about 10:10 a.m.
When officers arrived, they were shot at by a man on the second floor balcony of the building.
A squad car was hit, but no officers were injured. No children were shot, Police Chief Pete Nimmer said.
Officers did not return fire.
The chief said two men and one woman were killed, including the suspect.
Nimmer said they believe the suspect shot himself. He said that after the officers were fired upon, they thought they heard one more round soon after. They believe that was a self-inflicted gunshot wound to the suspect.
The names of the victims have not yet been released.
The person injured had non-life-threatening injuries, Nimmer said.