The suspects allegedly entered the Olympian gas station at 701 Harbor Blvd. at about 9:45 p.m. and approached the teenage clerk, who was alone in the store, according to police.
One suspect brandished a knife while the other two suspects grabbed the clerk and took him to a back room, police said.
After a few minutes, the clerk, who was not injured, left the back room and called 911, police said.
The suspects, whose ages are unknown, were wearing dark clothing and ski masks. They were last seen driving south on Old Country Road in a dark-colored car, police said.
The monetary loss from the gas station has not been determined. Anyone with information about the robbery is asked to call the Belmont Police Department at (650) 595-7400 or the Belmont Police Crime Tip Line (650) 598-3000.