The crash happened just after 7:30 p.m. PT between Kamloops and Merritt, near the Lac Le Jeune exit.
Investigators say the vehicle caught a soft shoulder, left the roadway and rolled five or six times before crashing 60 metres at the base of an embankment.
Police say witnesses, including a doctor and a nurse, provided first aid before emergency crews arrived.
Three people — a 49-year-old man and a 61-year-old woman, both from New Westminster, and a 41-year-old man from Surrey — were killed.
The 42-year-old female driver and a 70-year-old male passenger suffered non-life threatening injuries.
"They were very lucky they were wearing seat belts, which was fabulous and definitely added to their safety," said Cpl. Kathleen Thain.
Alcohol has been ruled out as a factor in the crash. Investigators said they have not yet determined if speed was a factor.