Robert "Robby" Richardson, 10, and Steven D. Smith, 46, died of blunt-force head and chest injuries at the farm 13 miles west of Reserve on Monday night, Sheridan County Coroner David Fulkerson said.
Barbara Smith, 71, was taken from the basement of her home to a hospital in Plentywood and was later transported to Billings. The extent of her injuries were not immediately known.
Nobody else was believed to be at the farmhouse when the tornado struck Monday evening, though the search is continuing with the daylight, Fulkerson said.
"Things are chaotic at the scene," he said. "As far as I could see, everything on the property was destroyed."
Don Simonsen of the National Weather Service in Glasgow said the tornado touched down west of Reserve between 7:15 p.m and 7:45 p.m. before crossing into North Dakota, where it weakened.
There also was at least one tornado reported about 20 miles south of Flaxville in Daniels County, Simonsen said.
The weather service sent a team late Monday to assess the damage.
Medicine Lake resident Brandon French said he and five other men were watching the tornado from a hillside.
"It was a damn big one. It was a long way away, but you could see it clear as day," French said. "It was like nothing we've ever seen before."
French said they got a call from the owner of a bar in Reserve who had heard that a farm to the west had been hit. The men got into a pickup truck and were among the first to arrive.
The house, barn, lean-to and various buildings at the farm were simply gone, he said.
"There's no houses, there's no buildings, there's nothing left," French said. "The vehicles were all turned over, there was a pickup plowed into a tractor - it just wrapped the pickup around the tractor. You couldn't peel it off."
Sheridan County Sheriff Patrick Ulrickson said the damage was devastating - the house was completely gone from the foundation. The tornado also destroyed all the other buildings on the property, he said.
"We had a Quonset hut that was crushed like a pop can," he said.
Todd McCabe also headed to the farmhouse after receiving a call from the bar owner in Reserve. He said he arrived as five or six different neighbors were trying to reach the basement.
With the house gone, the basement was just a hole in the ground covered with debris, he said Tuesday. The neighbors dug through rubble and found the woman, who he said was the teenager's grandmother.
Barbara Smith was conscious, but the boy next to her was still, he said.
"The lady was trapped down there and we just tried to get the stuff off from on top of her," McCabe said. "The boy was found right next to her. He was gone. The chimney fell on him and killed him."
Steven Smith lived in a trailer home on the property, which was also destroyed, Fulkerson said. Smith may have been trying to reach the basement when the tornado struck, though it was impossible to know for sure, he said.
Northeastern Montana is part of the Hi-Line, with vast stretches of plains and rolling wheat fields just south of the Canadian border. The area where the tornado touched down was "extremely isolated," Fulkerson said.
Sheridan County's population has been shrinking, with just over 3,200 people in 2008, according to the U.S. Census Bureau. The main source of income for its residents is farming.
In North Dakota, a sheriff's dispatcher in Divide County said they were no reports of storm damage or injuries.
Tornadoes are relatively rare in Montana, although on June 20, a tornado tore apart the state's largest indoor arena.
The June tornado tore off the roof and some siding at the Rimrock Auto Arena in Billings and caused extensive damage to the interior. The arena was not in use at the time.
The tornado was the first large tornado to hit Billings in more than a half-century.
Hundreds of households also suffered damage from severe winds or hail, city officials said.
Associated Press writers Thomas Peipert and Judith Kohler in Denver contributed to this report.