A state of emergency has been declared in Napa, Sonoma and Yuba counties, north of San Francisco Bay.
They are among eight counties affected by 14 major fires, which have burned through more than 49,000 acres and destroyed at least 1,500 homes.
There were mandatory evacuations for up to 20,000 people late on Sunday but some residents found that escape roads were blocked by downed trees and flames.
One of the deaths occurred in Mendocino County and two others were in Napa County. The other seven were in Sonoma County.
The two people who were seriously injured have severe burns and are in a critical condition in hospital.
According to the San Francisco Chronicle. more than 100 people have been treated for injuries, including burns and smoke inhalation, at hospitals in Napa and Sonoma counties.
In Napa County, fire chief Barry Biermann said the fires had burned more than 68 square miles but crews were struggling to control the flames.
He added: “Right now, with these conditions, we can’t get ahead of this fire and do anything about the forward progress.”
The largest city in the fire area is Santa Rosa, with a population of about 175,000. There, houses and business were ruined, schools were closed and two hospitals were forced to evacuate hundreds of patients.
A Hilton Hotel was among the buildings that were left in ruins.
Jeff Okrepkie, who had lived in his Santa Rosa home for five years, had fled but was later sent a photo of what remained of his home: a heap of burnt metal and debris.
He told Associated Press: “We live in the valley, where it’s concrete and strip malls and hotels and supermarkets.
“The last thing you think is a forest fire is going to come and wipe us out.”
Smoke was thick in San Francisco, around 60 miles south of the fire in Sonoma County.
California Department of Forestry and Fire Protection director Ken Pimlott said: “Imagine a wind-whipped fire burning at explosive rates.
“This is 50 miles per hour.
“Literally it’s burning into the city of Santa Rosa… burning box stores.
“This is traditionally California’s worst time for fires, California’s most damaging times for fires have occurred in October.”
Further south, there have been more evacuations in California’s Orange County due to a fire.
Television showed homes ruined by flames in the Anaheim Hills area, where at least 1,000 homes were previously evacuated.
That fire has grown to 2,000 acres and is being fought by 200 firefighters.