Su Oh mixed four birdies with two bogeys to keep herself in contention with a 70 to be in a share of fifth at 10-under, and best of the four Australians in the field.
Katherine Kirk also carded a 70 on Saturday to be at five under while fellow Aussies Hannah Green (73) and Minjee Lee (75) are well back at four over.
Kim led by as many as five shots with three birdies over the opening seven holes but she stalled on the back nine of Tiburon Golf Club as Korda made her move.
“I had a pretty solid round front nine,” Kim said.
“But back nine was little tough to focus. I kept thinking about future, so I was like, ‘Nope, Sei Young. Stop thinking. Focus on it’.
Korda began the back nine with three straight birdies, including a bunker shot she holed, and shot 31 on the back for a 66.
“I told myself to be really aggressive on the back nine, and it worked out today,” world No.3 Korda said.
Kim and Korda will play in the final group, along with Masson, with a cheque of $1.5 million ($A2.2 million) at stake for the winner.
World No.1 and LPGA player of the year Jin Young Ko moved up the leaderboard with a 66 to be in the group along with Oh and Jessica Korda (69), the oldest sister of Nelly Korda, six shots behind Kim.
Defending champion Lexi Thompson (70) made an early charge, only to be slowed by a pair of bogeys on the back nine to leave her in a group tied for ninth seven shots off the lead.
The format changed this year from a points-based system in which only the top 12 players had a shot at a $1 million ($A1.47 million) bonus to any of the 60 players who made it to the Tour Championship being in contention for the major prize by winning the tournament.