All three were veteran deputies. The department hasn't said what they were doing at the time of the blast on Friday or what caused it.
Los Angeles County Sheriff Robert Luna said the arson and explosives team undergoes in-depth training and responds to more than 1000 calls a year. The deputies had served 19, 22 and 33 years in the department, Luna said.
"They have years of training," the sheriff said at a news conference.
"They are fantastic experts and, unfortunately, I lost three of them today."
The explosion was reported about 7.30am at the Biscailuz Training Facility, Los Angeles County Sheriff's Department spokesperson Nicole Nishida said.
Aerial footage from KABC-TV shows the explosion happened in a parking lot filled with sheriff's patrol cars and box trucks. Three covered bodies could be seen near a truck with a ramp attached to a side door. A patrol cruiser parked nearby had its rearview mirror shattered by the blast.
Luna said it took more than four hours to render the scene safe and the deaths are being investigated by the department's homicide detectives, with the assistance of the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives. No one else was injured in the explosion, he said.
An early line of investigation was looking at a possible training accident, according to a law enforcement official briefed on the matter who was not authorised to discuss it and spoke to The Associated Press on condition of anonymity.
In a post on X, Attorney-General Pam Bondi said the explosion "appears to be a horrific incident" and federal agents are at the scene to learn more.
"Please pray for the families of the sheriff's deputies killed," Bondi wrote.
Luna said the deaths marked the department's worst loss of life in a single incident since 1857, when four officers were killed by gunfire, and noted that he couldn't release the deputies' names because he had yet to speak to one of the families.
"I have met with two of three families thus far. Those were extremely challenging conversations," Luna said, his voice breaking.
Arson investigators from the Los Angeles Fire Department and members of the Los Angeles Police Department bomb squad were also assisting the investigation at the training facility, Los Angeles Mayor Karen Bass said on X.
