On 20 January FamilySearch added an indexed dataset, the Canada Census Mortality Schedules, 1871.
Although relatively small, only 45,371 people, it's a unique record set only available online at FamilySearch.
The 1871 census of Canada was nominally taken on 2 April, 1871. "Honest, intelligent, well-instructed and painstaking" enumerators visited each dwelling to record information; the process took a few weeks. They were instructed to enquire of every family whether any death has occurred in the family or house during the last twelve months, including deaths of newly-born infants, and if so enter information in Schedule 2, Return of Deaths.
Of the death reported 3,568 were for infants born in the census year (January - 2 April); 9,437 were born the previous year; 3,342 in 1869; 2.159 in 1868; and 1,168 in 1867.
At the other extreme are two deceased who passed away at the wonderful age of 120.
Jane McKay's entry is shown in the image. There is also an Ontario death registration showing the same age. Had I found a burial registration giving the same age would you accept it as meeting "the rule of three?"
The other is a man named only as Mosquito from the Tuscarora (Indian Reserve) e, South Brant 15, Ontario, with no corresponding Ontario death certificate.