At least two other people were injured as the gunman fired randomly on other vehicles on a freeway, before he shot and killed himself near the city of Orange, south of Los Angeles.
The suspect was identified as 20-year-old Ali Syed, who was unemployed, had no criminal record, and was living at home with his parents and taking a class at a local college.
In all there were three murders and five attempted murders. The motive for the attacks was being investigated, said Orange County sheriff's spokesman Jim Amormino, in an afternoon update on the early morning shootings.
He described Syed as "kind of a loner (who) doesn't have a lot of friends."
The first killing was a woman found dead in the early hours in Syed's family home in Ladera Ranch. The woman was not known to his relatives, and police said they were working to identify her.
Syed then fled in an SUV, and "multiple incidents" occurred as he drove to nearby Tustin, about 35 miles southeast of Los Angeles, said Tustin police department spokesman Paul Garaven.
In one of the first carjackings, Syed appeared "very polite" as he walked up to a gas station customer.
"I don't want to hurt you. I killed somebody and today's my last day. Give me your keys," he told the customer, according to Tustin police chief Scott Jordan.
He then took the man's car and drove onto a nearby freeway, where he stopped and opened fire on passing vehicles, trying to get another car.
In another carjacking, Syed got out of his vehicle and confronted a motorist in a BMW.
"He orders him out of the vehicle, walks him to the side of the curb, and executes (the) victim," said Santa Ana Police Corporal Anthony Bertagna.
Amormino said four people were confirmed dead including the gunman. "With each carjacking there was a shooting involved," he said.
Amormino added that there was no active pursuit of the suspect's vehicle until it was located near Orange, where the gunman took his own life. "He stopped the vehicle and got out and shot himself," he said.