I live in North pole Alaska, interior.
When I googled it showed your usual lows as just -20F. I thought it got down to -40F and such in North Pole???
Anyway... if your lows are usually about -20F (usually, ignore short drops), then you don't have to stress that much.
Just read through the article that has previously been mentioned (and is in my signature line. If on a phone turn the phone sideways).
If you get steady temps way below -20F in the winter, I would suggest making two coops, one for winter and one for summer.
Coop 1. A way bigger summer coop, or you could have a summer only tractor that is moved every few days. A tractor can have much smaller square footage since the chickens are distracted by new grass and bugs. Moving the tractor means they always have a green play space instead of boring dirt only.
Coop 2. Your winter coop. For only 3 hens, if you do NOT run your cars in the garage, they could be kept in a garage coop. They could stay in a large roomy, walk in summer coop, and then if/when a stretch of -40F is forecast, bring them into a smaller garage coop until the temperatures are -20 or warmer.
I am guessing the garage stays around 0F? If your garage is heated, you don't want to bring them from 50F to -20F... that is a bit harsh of a drop.
For a temporary garage coop, that is used for no more than a week at a time, you could probably get by with 4 square feet per chicken of coop space (so coop needs to be 3x4 or 4x4), and then since the run would only be for a week, a 5x5 run would do.... but bigger would be much better. With such a tight space you probably will also need to supply them with distractions... leaves, hay, veggies, crickets from the pet store....
Also, if they are there for longer than a week, it needs to be bigger.
You will also have to figure out ventilation issues.
Remember that a small closed up space with little ventilation will:
1. Make them sick, give them respiratory infections
2. Increase the risk of frostbite due to higher humidity
3. Greatly increase the risk that they start to eat each other...literally