The occupants of the three vehicles suffered minor to major injuries in the 2:35 p.m. accident, Paulson said.
Two occupants of a Porsche Carrera suffered significant but not life-threatening injuries and were taken to Queen of the Valley Medical Center in Napa. The roof of the Porsche had to be cut away, Paulson said.
A Toyota Camry spun into a guardrail and caught fire. The solo occupant suffered moderate to major injuries and also was taken to the Napa hospital, Paulson said.
The solo occupant of a Mitsubishi Montero was not seriously injured and the driver of the big rig truck was not injured, Paulson said.
The big rig slid sideways as it dumped its load of boulders, some at least 2 feet in diameter, along the two-lane road a mile from the Sonoma County border, Paulson said.
State Highway 121 from Napa Road to Duhig Road remains closed in both directions. Paulson said a big-rig tow truck is on the scene and the CHP hopes to begin one-way traffic through the area by 6 p.m.
A Sig-alert issued at 2:55 p.m. remains in effect.