The Canadian cable news channel CP24 reported several occupants of the lower unit were not home when the fire broke out shortly after 12 a.m.
One of the tenants of the first-floor unit said she came home to "chaos."
"There were police cruisers everywhere, fire trucks everywhere, people crying," said the woman, who identified herself as Carley. "It was devastating. Everything was in ruins."
The Canadian Broadcasting Corp. reported the victims were rushed to a hospital where they were later pronounced dead.
The cause of the fire had not been determined and the victims' names were being withheld pending notification of relatives. CTV News said police believed the victims were in their 20s.