Ontario Provincial Police said the accident happened just after 11 p.m. ET on Dufferin County Road 10, about 60 kilometres northwest of Toronto.
Police say one person was pronounced dead at the scene while the other two died later in hospital.
Const. Paul Nancekivell said both cars — a sedan and an SUV — were found upside down several hundred metres apart.
“The accident was so severe it’s very difficult to tell what the vehicles are like,” he said.”
“It’s definitely going to take a while [to determine what happened]. The vehicles are barely recognizable. You would not be able to tell what kind of vehicles they are. They are very badly damaged, completely destroyed.”
Nancekivell said police haven’t located any witnesses to the crash.
Dufferin County Road 10 will remain closed at Mono-Amaranth Townline for much of the morning while police conduct their investigation.
Commuters are asked to use Highway 10 or County Road 11 as alternate routes.