Growing Arnica montana in Zone 3.

Is it feasible to grow Arnica montana in zone 3 in southern Manitoba?

The European arnica, Arnica montana, is "officially" rated hardy in zones 6 to 9, but hardiness ratings for herbs are frequently inaccurate. The main reason is that the zones are based solely on average minimum temperatures while factors such as drainage, snow cover, mulching, etc., can have a large impact on a plant's ability to survive the winter. The curator of the Devonian Botanic Garden in Alberta has told us that many herbs not rated to be hardy in northern Alberta thrive there.
Having made that point, it is unlikely that you can "push" zones enough to get Arnica montana to grow reliably enough as a commercial crop. Often it is feasible to get plants to survive in colder zones, but "pushing" the zones by three is too much. You might be lucky to get the crop to survive one or two winters out of three, but the third winter could destroy your crop and profits.

Is the market for American arnica as strong as for arnica montana?

The market for Arnica montana is more established. But because A. montana is a protected species in Europe supplies are limited and hence the demand for the American species, A. chamissonis, is getter stronger. I don't think that it is as strong as A. montana, but we do not have reliable market data available to verify that.