The RCMP said a dozen people were injured in the slide but up to 200 people were on the mountain at the time for the annual Big Iron Shoot Out rally.
The slide struck around 3:30 p.m. local time, and search and rescue teams were called in from around British Columbia and from Calgary.
The RCMP said they did not yet have details about the three dead or the extent of the injuries. Nor did they know how many might be missing in the slide near Revelstoke, about 300 kilometers (185 miles) west of Calgary and about 400 kilometers (250 miles) northeast of Vancouver.
"The area has been shut down to keep people out," RCMP Cpl. Dan Moskaluk said. "They're going to continue on here with trying to remove people off the mountain in the area and tomorrow reassess where we're at."
"There's air assets in the form of helicopters ... RCMP police dogs, services dogs, avalanche trained dogs, a variety of machines and people, essentially it's a large scale rescue that's being coordinated," Moskaluk said.
Kathy Berlingette, owner of Smokey Bear Campground Resort in the area, said the event was in a remote place and everyone involved had to drive their snowmobiles out to get there.
The Canadian Avalanche Center issued an avalanche warning for the region, which includes Revelstoke, for Saturday and Sunday, after a powerful storm blanketed the region with snow.
Greg Johnson, from the avalanche center, said at the time that there was danger that the snow would overload weak layers in the upper snowpack.
There have been a few avalanche deaths in the British Columbia backcountry this season but nothing compared to last winter, when there were two dozen deaths. There were 13 avalanche deaths the previous winter.
Revelstoke Mayor David Raven told CTV Newsnet that the gathering was an unsanctioned event.
"The Canadian Avalanche Center based in Revelstoke has had a warning for the last three weeks expressing extreme caution in the backcountry, a fresh snowfall overnight exacerbated that warning, I know people have been cautioned again and again," Raven said.