The incident happened in Toronto's east end on Thursday (local time).
Police also evacuated a building in the city's downtown later in the day due to a suspicious package in an incident related to the deaths, Detective Mike Carbone told a news conference.
He gave no details on how it was related or what the package contained.
In the initial incident, police responding to a call about a stabbing found three people who appeared to have been injured by crossbow bolts, said another police spokesman David Hopkinson.
The two men and a woman were pronounced dead.
"We don't have any idea with regards to why this may have happened," Hopkinson said.
CTV News, citing emergency services, said two other people were seriously injured.
A 35-year-old undentified man is in custody, police said.
Television footage showed police tape surrounding part of a residential street in Scarborough, a suburban area east of the city's downtown area.
In 2010, a man shot his father in the back with a crossbow in a Toronto public library before smashing his skull with a hammer. Zhou Fang, who had suffered domestic abuse, was convicted of a lesser charge of second-degree murder.
