Sheriff's deputies responded to reports of a shooting at a single-family residence in the 500 block of Stanford Avenue at about 1:30 p.m., San Mateo County Sheriff's Lt. Marc Alcantara said.
The three victims were found suffering apparent gunshot wounds and pronounced dead at the scene, according to Alcantara.
A motive for the shooting has not been established and no suspects have been arrested, Alcantara said.
"The investigation is still in the preliminary stages," Alcantara said, and added the incident is being investigated as a homicide.
Police are searching for a light blue Honda that was reportedly seen leaving the house at a high rate of speed shortly after the shooting, according to Alcantara.
Anyone who might have information regarding the shooting is asked to call the San Mateo County Sheriff's Office tip line at (800) 547-2700.