The explosion occurred about 12 p.m. at a textile business located in a row of commercial structures, the Los Angeles Fire Department said in a statement.
The victims were described as three males, two of whom were severely burned, the LAFD said. The third patient was in fair condition after suffering unspecified injuries.
The Fire Department declared a "major emergency" after 137 firefighters responded to the scene and were in a "fully defensive mode."
Just over an hour after the blast, the LAFD announced that the fire was extinguished. No other buildings were seriously damaged.
The cause of the incident is under investigation.