Franchitti started fourth but couldn't get around Castroneves or Briscoe, who formed the perfect blockade.
"Now we have a legitimate shot at the championship," said Power. "Two more road courses, two more ovals and we can get this."
Power has a mixed history at this Northern California track. In 2009 he had to be airlifted out by a helicopter after running into the stalled car of Nelson Phillipe. Power suffered a broken back in the accident and there was some question whether he would ever race again.
He came back strong in 2010 and won both the pole and the race before repeating the feat this year.
"It's one of the tracks I seem to click with," Power said. "It was very comforting to have Helio and Ryan behind me on the last restart, two guys you can totally rely on."
Unlike two weeks ago in New Hampshire when a late accident involving Danica Patrick led to a controversial finish - and resulted in an angry Power making an obscene gesture toward race officials that later earned him a US$30,000 fine - the ending was without much drama.
Power got the jump on Castroneves on the double-file restart, then ran comfortably ahead the rest of the way while cruising to his 14th career IndyCar victory.
As he made his way to the winner's circle, Power stopped to make another gesture with his hands.
"I gave it two fingers up," he cracked. "Not the middle ones, though."
The win has set up a wild run for the championship after Franchitti appeared to be comfortably ahead in the points race six weeks ago.
"There's going to be days like this," said Franchitti, who finished fourth ahead of Target Chip Ganassi teammate Scott Dixon.
"I was pushing as hard as I could just to keep the Penske cars in sight. I was really on the ragged edge for pretty much the whole day really."
Power held off a furious charge from Castroneves following a caution with nine laps to go.
Briscoe said teamwork was the key to the day.
"We sort of got to a point in the race where our focus was just to hold off Dario," said Briscoe, the only other driver to lead a lap.
"We started 1-2-3 and we finished that way. Right now for this team to win the championship we need to keep Dario behind Will."
It's the first IndyCar sweep for Team Penske since Paul Tracy, Al Unser Jr. and Emmerson Fittipaldi placed 1-2-3 at Nazareth in 1994.
It's also the first sweep of the top three positions by any IndyCar team since Andretti Autosport took the top four spots at St. Petersburg in 2005.
"It's invaluable to have guys like that performing at the top level, taking points away from Dario," Power said. "I expect them to be the same the next four races now that we've got a bit of momentum."
- AAP
