The accident occurred at 2:10 a.m. local time (0910 GMT) when a train carrying 40 to 60 grain hopper cars derailed near Field, British Columbia, the TSB spokesman said.
TSB is investigating the cause of the derailment, the spokesman added.
Canadian Pacific did not offer an immediate comment.
The three victims are believed to be a locomotive engineer, a conductor and a conductor trainee,” said François Laporte, president of the Teamsters, a union representing Canadian rail workers.
Eight railway workers have died in Canada since November 2017, including Monday’s deaths, Teamsters said.
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