The house in rural Hartland, about 25 miles northeast of Buffalo, burned to the ground, and investigators might never know exactly what started the fire, said Niagara County Fire Coordinator Jim Volkosh. He added the cause appears to be accidental.
The fire was reported shortly after 11 p.m. Monday and the house was engulfed in flames when sheriff's deputies and volunteer firefighters from six area companies arrived.
They tried to rescue the boys but the smoke and flames were too intense, Volkosh said.
The boys' grandmother, 59-year-old Leona Heigl, was listed in critical condition Tuesday at Erie County Medical Center in Buffalo, due to smoke inhalation and burned lungs.
All three boys were trapped in the downstairs part of the house, firefighters said.
Police identified them as 8-year-old Devon and 6-year-old Cameron Jenkins, and their 5-year-old brother Daydon Alston.