Two others survived the plunge and were airlifted to the Mercy Medical Center in Redding, Ca., said Brian Harris, a the U.S. Forest Service spokesman.
Their conditions were not known. The dead, ages 51, 29 and 19, included two men and a woman.
The crew was helping with a 500-acre backfire set to help contain another 1,350-acre fire 10 miles south of this northern California town when one wheel went off the narrow gravel road and the truck rolled off a steep ravine at about 1:30 a.m., Harris said.
``Indications are they rolled in the worst possible place. It's safe to say they rolled the entire 800 feet,'' over rocky and partially wooded terrain, Harris said.
Meanwhile, in west-central Oregon, firefighters ordered the evacuation of the 4,000-5,000 people after stiff wind fanned a 3,300-acre fire near a subdivision just west of Sisters.
Wind was causing spot fires in and around the Black Butte Ranch resort, where one home was reported burned. The complex is made up of two golf courses, a lodge, about 1,300 homes, not all of which are occupied year-round.
Black Butte general manager Loy Helmley said it would be a few days before residents can return. The Red Cross was setting up evacuation centers.
A sprawling wildfire near the Columbia River port town of The Dalles singed another 3,000 acres overnight and burned to within inches of some buildings but had not destroyed any homes by Sunday.
``The fire is continuing to pose serious control problems,'' as gusty winds periodically kick up the flames, said Peg Foster, an Oregon Department of Forestry spokeswoman.
Crews had the fire about 55 percent contained, she said. Residents of 250 homes have been urged to evacuate since the lightning-started fire flared Thursday.
Stiff winds whistling through the Columbia River Gorge pushed the fire toward high-tension electrical wires overnight, Foster said.
The wires were still live Sunday, but if carbon-rich smoke thickens around the cables, it could cause the electricity to arc, endangering firefighters. If that danger intensifies, she said, authorities might switch them off and reroute power.
In California's Sierra Nevada, a blaze in and around Giant Sequoia National Monument had grown to 66,000 acres Sunday, after burning an additional 1,600 acres during the night.
The ancient sequoia trees weren't completely safe but firefighters had minimized the threat, said fire information officer Jill Slater.
``They're really getting a handle on it,'' she said.
The wildfire was 30 percent contained.
A woman was arraigned Friday on charges of starting the fire, about 130 miles north of Los Angeles, while cooking over an illegal campfire.
Thirty-one major fires still active on Sunday around the West and in Alaska had burned about 491,000 acres, the National Interagency Fire Center reported.