According to fire department spokeswoman Mindy Talmadge, one worker suffered burns to his hands and two others were hospitalized for smoke inhalation.
The accident took place at about 1:50 p.m. in the first floor office of the multi-story building at 750 Van Ness Ave.
The building's automatic sprinkler system went on immediately and firefighters were continuing to flush the office out with water this afternoon, according to Talmadge.
None of the residents of the building had to be evacuated, Talmadge said.
Pacific Gas and Electric Co. and California Occupational Safety and Health Administration representatives were on the way to investigate at about 3 p.m.
Northbound Van Ness Avenue between Golden Gate Avenue and Eddy Street was closed following the accident, backing up traffic. The roadway was reopened at about 2:50 p.m., with one lane in front of the building remaining blocked by fire trucks.