The bodies were discovered in a home on the 700 block of Sierra Vista Court near Glendora Avenue.
Detectives say the call came in at 10:23 a.m. Wednesday from neighbors reporting a man with a gun.
"When deputies arrived, they found three victims suffering from gunshot wounds. All three victims were pronounced dead here at the scene," said L.A. County Sheriff's Homicide Lt. Eddie Hernandez. "All three victims were dead at the scene."
Shortly after the shooting, authorities placed three young children in an unmarked car.
Investigators say the kids were not in the house at the time of the shooting, but wouldn't say how they were involved in the case.
They did say that it appears the shooter was the former boyfriend of the woman who lived there.
"It appears that there was a relationship that went estranged, and he decided to show up today and take his ex girlfriend's life, potentially her new boyfriend, and his," said Hernandez.
A friend of the deceased woman says the woman's ex-boyfriend hadn't been back to the house in several months. But when he did show up, the two would argue.
Officials say all three deceased were in their 40s. Their identities were not released.