Lang is tied with South Koreans Shin and Park Sung-hyun at 11 under at the demanding Aviara course in Carlsbad.
Ko was two-under for her front nine then reeled off three straight birdies from the 11th to put her in a strong position to win her 11th LPGA Tour title.
"You don't get that many opportunities where you're going into the final round with a three-shot lead,” the 18-year-old said. “But I'll take it. I'd rather be a couple shots ahead than a couple shots behind.”
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Park kicked off a run of five consecutive birdies at the 12th but an errant tee shot on 17 led to a double bogey. She hit back immediately though with a birdie on the final hole to stay in touch.
Australia's Minjee Lee provided the highlight of the day when her 5-wood tee shot at the 275-yard par-4 16th landed in front of the green, kicked left and rolled into the cup for the second hole-in-one on a par-4 in LPGA history.
The first came earlier this year from South Korean Jang Ha-na in the Bahamas.
"Just lucky," said Lee, adding that it was her first hole-in-one. "I hit a good one shot, though. It's a bit of both.”
World number two Park In-bee is four strokes off the pace on 10-under with Jessica Korda a shot further back.
(Reporting by Tim Wharnsby; Editing by Peter Rutherford)
