M+S will be OK in Western Canada
I assume that you will be travelling up to Western Canada, the Prairies, or BC, as opposed to Eastern Canada and more specifically Quebec.
If you were travelling in Quebec, that Province now requires tyres with the snow flake symbol in the winter months. I doubt this applies to "out of State" vehicles, but I do not know.
Elsewhere, regular all season, (well even summer), tyres are legal. On the highways in the Rocky Mountains, for the most part, summer tyres in the winter months are not OK, but All Season or M+S is OK.
I run Dunlop M+S 18" on my 3 in winter here in Alberta which is much like Montana.
Chains, you will not need, as if you did, the roads are already blocked with spun out semis. If you wait until about 10:00 am before you get on the road in the mornings, for the most part, the roads have cleared somewhat from the morning traffic so tend to be OK. You can drive till about 10:00 PM usually before the roads start to get slippery again, but stay off them early in the morning.
If there is a blinding snow storm, find a motel, go to the bar, and wait it out. The next morning after a storm is most often always bright, sunny, and crisp - a pleasure to drive and often free of all but cars still in the ditch from overnight. Driving ahead of the snowplows on fresh fallen snow is very quiet and actually quite pleasant in a full time AWD. If it is cold enough, the snow in the trees will be sparkling like a thousand jewels.
The 3 has a great heater(s), (at least the cold climate spec ones), so you will be nice and warm; the defrost fan works well and if you have the electric heated windscreen, you will be able to use it in the mornings. Also bring along a corn broom to sweep the snow off.