New South Wales Premier Gladys Berejiklian confirmed the deaths in the state's Snowy Monaro region, which came as Australia grapples with an unprecedented fire season that has left a large swath of destruction.
Canada-based Coulson Aviation said in a statement that one of its Lockheed large air tankers was lost after it left Richmond in New South Wales with retardant for a firebombing mission. It said the accident was "extensive" but had few other details.
"The only thing I have from the field reports are that the plane came down, it's crashed and there was a large fireball associated with that crash," Rural Fire Service Commissioner Shane Fitzsimmons said.
He said the crewmen were aged 42, 43 and 45.
"We will be forever indebted to the enormous contributions and ultimate sacrifice that's been paid by these extraordinary individuals," Fitzsimmons said.
Foreign Minister Marise Payne said she had conveyed Australia's condolences to U.S. Ambassador Arthur Culvahouse Jr.
"Our hearts go out to their loved ones. They were helping Australia, far from their own homes, an embodiment of the deep friendship between our two countries," she said in a statement.
Payne added: "Thank you to these three, and to all the brave firefighters from Australia and around the world. Your service and contribution are extraordinary. We are ever grateful."
The tragedy brings the death toll from the blazes to at least 31 since September. The fires have also destroyed more than 2,600 homes and razed more than 10.4 million hectares (25.7 million acres), an area bigger than the U.S. state of Indiana.
The three were part of a crew on a California-based tanker, the governor of that state said in a statement on Thursday. California Gov. Gavin Newsom did not identify the crew members but called them heroes.
Coulson grounded other firefighting aircraft as a precaution pending investigation, reducing planes available to firefighters in New South Wales and neighboring Victoria state. The four-propeller Hercules drops more than 15,000 liters (4,000 gallons) of fire retardant in a single pass.
Spokeswoman Robyn Baldwin of Coulson, with headquarters in the Canadian province of British Columbia and extensive U.S. operations, declined to identify the crew members or say what U.S. states they were from.
"We ask for privacy at this time as we mourn the loss of our crew members," Baldwin said.
Australian Transport Safety Bureau, the national air crash investigator, and state police will investigate the crash site, which firefighters described as an active fire ground.
"There is no indication at this stage of what's caused the accident," Fitzsimmons said.
Berejiklian said there were more than 1,700 volunteers and personnel in the field, and five fires were being described at an "emergency warning" level — the most dangerous on a three-tier scale — across the state and on the fringes of the national capital Canberra.
Also Thursday, Canberra Airport closed temporarily because of nearby wildfires, and residents south of the city were told to seek shelter. The airport reopened after several hours with Qantas operating limited services, but Virgin and Singapore Airlines canceled flights for the rest of the day.
The blaze started Wednesday, but strong winds and high temperatures caused conditions in Canberra to deteriorate. A second fire near the airport that started on Thursday morning is at a "watch and act" level — the middle of the three tiers.
Residents in some Canberra suburbs were advised to seek shelter and others to leave immediately.
"The defense force is both assisting to a degree and looking to whether that needs to be reinforced," Chief of Defense Angus Campbell told reporters.
"I have people who are both involved as persons who need to be moved from areas and office buildings that are potentially in danger, and also those persons who are part of the (Operation) Bushfire Assist effort," he said.