The provincial government says the week-long inquest into what happened at St. Mary's Villa in Humboldt in December 2010 is to begin Monday in Humboldt.
Staff initially thought seniors were getting sick because of a stomach virus or food poisoning.
Workers were getting splitting headaches, but they never thought gas poisoning was a possibility.
There were no carbon monoxide detectors in the home.
An 89-year-old priest died an hour after an evacuation of the home began and two other elderly women died in the weeks that followed.
An inquest looks at the events surrounding a death and a coroner's jury can make recommendations to prevent similar deaths.