The Ryder Cup rookie had a two-under 70 and is holding off a pair of players who are No.1 in the world.
One shot behind was Jackson Koivun (68), the leading amateur. Another stroke back and far more daunting was the top-ranked Scottie Scheffler, closing hard with a 64 that included a double bogey at Silverado on Saturday.
"Not too worried about what he's (Scheffler) going to do. I've just got to play better than him," Griffin said.
"And if I do that tomorrow, should be able to lift the trophy. Obviously there's some other guys there in the mix, too, but I think everyone knows what Scottie Scheffler is capable of and I've just got to stay in front."
Griffin, who started out with a three-shot lead, is 16 under and will be in the final group with Koivun, who will try to become the second amateur in as many years to win on the PGA Tour, after Nick Dunlap triumphed in La Quinta.
"Didn't have a great start, not the start I was looking for, but was able to kind of turn it around midway through the round, make some birdies, make some putts, kind of flip the script and keep myself in contention," Koivun said.
Koivun has already done enough in the PGA Tour University system to earn a card when he turns pro.
And then there was Scheffler, finding a comfort zone with each day on a Napa course he had never seen until Tuesday.
He missed only two greens, one of them at the 14th when he drove into the rough, came up just short and tried to clip a wedge to a tight pin. It came out soft and didn't clear a bunker, and he failed to get that up-and-down, taking double bogey.
Scheffler answered with three birdies on his last four holes, two of them par 5s, the other a seven-iron he hit to six feet on the par-3 17th.
"If I go out and play good, consistent golf, I'm going to start moving my way up the leaderboard and that was the goal going into today," said Scheffler, who started the round eight shots behind Griffin.
Karl Vilips, the only Australian left in the field, is two under after his 71.
With AAP.
19°C