The deadly incident, at Farmers Fresh Mushrooms Inc., a mushroom supplier for western Canada and the United States, happened late Friday when a group of workers entered a utility shed, the Royal Canadian Mounted Police said.
Other workers who came to the men's aid also were overcome, police said.
The three initial workers died at the scene, police told the Canadian Broadcasting Corp.
It was not clear if the toxin release was accidental, police said. Several people working at the site "either accidentally or intentionally released" a toxic chemical used in composting, said Constable Holly Marks of the Langley Mounties.
Two hospitalized workers were in critical condition, while the third was listed as stable, the CBC said.
WorksafeBC, formerly the provincial Workers' Compensation Board, and the British Columbia Coroners Service were investigating.