The incident happened around 4 a.m. on the Highway 427 northbound ramp to the westbound Highway 401, according to the OPP, when a Honda Accord struck the right side of the guardrail.
Two people were found dead in the car. Police have not released their age or gender.
Moments after the crash, passersby also pulled a third person, a woman, from the car before it was fully engulfed in flames. She was taken to hospital and was later pronounced dead. Police have not released her age.
Investigators are looking into the circumstances of the crash, OPP said.
The roads remained closed Saturday morning for investigation and traffic has backed up, said OPP Sgt. Kerry Schmidt. It is unclear when the roads are slated to reopen.