The fire started Friday morning north of Neola, about 100 miles east of Salt Lake City, and on Sunday morning crews had it about 5 percent contained. The cause had not been determined.
The fire began spreading into the Ashley National Forest on Sunday afternoon, prompting federal authorities to close the forest to public use.
The Federal Emergency Management Agency said about 300 people are under a mandatory evacuation. Another 150 homes could be threatened by Tuesday, said Derek Jensen, a FEMA spokesman.
A U.S. Forest Service command team that travels the country fighting the largest fires joined local crews Sunday and took over direction of the firefighting efforts.
Eleven-year-old Duane Houston escaped the fire but his father, 43-year-old Tracy Houston, and his grandfather, 63-year-old George Houston, were killed by the flames Friday as they worked in a hay field, authorities said. The owner of the field, 75-year-old Roger Roberson, died at a hospital, officials said Saturday.
In Montana, fire managers on Sunday were reducing personnel assigned to a blaze that had burned nearly 6 square miles in the Gallatin National Forest. Evacuation orders remained in effect Sunday for several dozen summer homes, the Madison Arm Resort, some campgrounds and a ranger station.
In California, a wildfire that destroyed at least 254 homes south of Lake Tahoe was 95 percent contained late Sunday, the U.S. Forest Service said.
Crews also battled a wildfire Sunday that had blackened more than 482 acres of brush in a rugged area of Santa Barbara County near popular campsites and swimming holes.
That fire, which started Saturday evening, was 60 percent contained early Monday, U.S. Forest Service spokesman Robert Rainwater said. The blaze had closed some campgrounds, but no residents had been ordered to leave their homes, he said. The cause of the fire was under investigation.
A pair of wildfires also burned near the town of Julian about 60 miles east of San Diego. The larger of the two had consumed 110 acres and was 20 percent contained by Sunday night, authorities said. The other blaze to the west blackened about 60 acres.
Firefighters in Los Angeles County fought a 50-acre blaze near Pyramid Lake on Sunday before it was contained in the early evening. The blaze was ignited by a vehicle that caught fire, officials said.