Can three-season tents be used in the snow?

A: Sure they can. Three-season tents and four-season tents don't differ all that much, at least in terms of structural integrity. Same pole strength, basically same fly and canopy materials. A true four-season tent will tend to have the following characteristics: Less mesh in the canopy, for a little more warmth; a larger vestibule, for storing gear and perhaps donning boots; more interior space, for bulkier winter clothing and sleeping bags; in some cases, an additional pole for extra resistance to snow loading.

The biggest real problem, potentially, is the extra ventilation. But face it, tents are hardly insulated devices anyway, so it's not as if there will be a huge difference, temperature-wise, between a tent with a bunch of mesh and a tent with not so much. Some, difference, true, but in most cases acceptable. In any event, you can make a three-season tent more winter-friendly by digging it down into the snow, to cut the wind, and to bury the lower edge of the rain fly in snow, so that the fly itself provides a good, wind-proof shell for the tent. Without a shred of empirical evidence to back it up, I've also long felt that laying a "space blanket" on the floor of the tent, reflective (silver) side up, helps keep the tent a little warmer.

Alternatively, you can take the tent as a backup shelter and practice your igloo building or snow cave digging. Either makes a far better winter shelter than any tent around—warmer, quieter, and wind-proof. Plus the process of making one helps pass the time while generally proving highly entertaining.