South Korean rookie Mirim Lee was second after a 67. Norway's Suzann Pettersen was another stroke back after a 69.
Cristie Kerr matched the low round of the tournament with a 64 to jump 44 spots to a tie for 12th at 5 under. She hit 10 of 13 fairways and had only 26 putts.
Park, using a blade putter instead of a mallet for the first time since 2008, birdied the first hole and stayed in front until Pettersen birdied Nos. 7 and 8 to tie for the lead. The tie lasted one hole before Park rolled in a 12-foot birdie putt at the ninth.
Pettersen birdied the par-4 13th with a 20-footer to tie again, only to have Park roll in the 25-footer from the fringe at 14. Pettersen then matched Park's bogey at 15, also missing a 3-foot par try.
That's when Lee slipped in between the two with birdies at Nos. 15 and 16.
Pettersen, who for much of last year was No. 2 in the rankings to Park's No. 1, said she struggled with her game, gusty wind and greens that are firming up.
"Overall it was one of those days where you just fight to stay in it," said Pettersen, ranked fourth in the world. "I didn't have the greatest of feels throughout the round. There were a lot of challenges out there, and there will be as many tomorrow and hopefully I can do just a little better."
Park, who shot 66 in each of the first two rounds, said it was easily the toughest day of the three.
"The wind was quite tricky to me," she said. "I played pretty consistent again, but the golf course was tougher, the conditions harder. I have to stay consistent and keep rolling in some putts."
Park knows very little about Lee.
"I know she's from Korea and she played on the KLPGA Tour, and I know she's hitting it quite long from seeing it from behind today," Park said. "Except for that, I really don't have much information. Obviously she played quite solid for three days."