The fire broke out at about 11:45 a.m. at 285 Park Lawn Road, north of The Queensway.
About 12 fire vehicles responded, along with 45 firefighters.
Firefighters believe the fire started in a second floor apartment in the living room. They say the man who lives in that unit was taken to the hospital with serious burns along with two other people.
One other person was treated at the scene.
Firefighters also had to help one family to escape by ladder.
"A lot of people were in one apartment. They had pets and they were kind of panicky about the smoke, "said fire chief Don Beam.
"The smoke was too heavy for them to go into the hallway — and I think they had actually opened the door and let a lot of smoke into the unit — so a couple of the aerial crews put ground ladders up to the third floor and brought the people down with their pets and pet cages," he said.
The fire was quickly brought under control.
The cause of the fire is still being investigated.