The shooting took place at 00:45 a.m. local time (0745 GMT) in the 8100 block of Leeds Street in Downey, a city located in southeast Los Angeles County, police officials said in a statement.
The officers found four men and a woman had been shot, the report said.
Three men were pronounced dead at the scene, it said, adding that the injured were transported to a hospital.
The circumstances that prompted the shooting were unknown, but authorities described it as an "isolated incident," Xinhua reported.
Further details are awaited as investigations are under process.
ZZ/PR