Assuming you could cast a few spells before the avalanche fell, and the rocks in the avalanche aren't tiny, you could cast a few Force Ladders to block the rocks. Of course your caravan would have to get through within 5 rounds.
If you are nearby Leomund's tiny hut, might work. I'm not sure exactly what it can keep out, an avalanche might be too much force and would collapse it.
Other than that, I can't think of any spells of 3rd level or lower.