Mud, possibly produced by recent heavy rain, slid across part of State Route 530 near Oso, Washington, north of Seattle, Washington state trooper Mark Francis said.
The three deaths were confirmed by the Snohomish County Sheriff's Office, which also reported rescue crews using equipment and helicopters were still on the scene some four hours after the mudslide. At least six houses were destroyed, the Sheriff's Office said in a statement.
Five people were brought to the Cascade Valley Hospital in Arlington, Washington, and more people were expected to be transported from the slide area, said hospital spokeswoman Jennifer Egger.
"It's still a very active rescue scene," Egger said.
She said she was unable to discuss the extent of the injuries.
Francis said the slide was in the foothills of the Cascade Mountains where rains have been heavy lately.
The mudslide caused blockage of the Stillaguamishi River, adding flooding to the mud problem, Francis said.