You can't keep it warm but you can conserve heat as much as possible. You're supposed to add extra insulation by covering the windows with blankets, pick a space in the house where everyone will be and additionally insulate that main space from the rest of the house, use candles as it's safe to.

Our houses are generally quite well insulated due to the climate. Government preparedness info says a modern house (~15 years old) can go four full days in -20C weather before inside temperatures drop to 5C. A typical 70s row house would drop to that after 48 hours but a 70s brick house can drop that low after just 24 hours. So depending on your house and location, the specific plan for a 3-day outage can range from "you'll be fine as long as you have blankets" to "you need a fireplace or other heater with fuel for at least a few hours a day".

In Canada, things called "storms" sometimes knock out power lines. The result is no power!

I've been without power for days in the cold, and have been compelled to put on my coat... inside! Most Canadians do this, rather than trying to keep an entire house warm at -40C.

It's really a non issue.

Worried?

* Buy a few large candles. Just one provides a lot of warmth if you cup your hands around it.

* Eat more. You need 2x your calories when it is cold. Food means life in the cold.