Two-time major champion Chun matched Yin's 65 at the SKY72 Golf and Resort in Incheon, west of Seoul, while LPGA rookie Park, who won this year's U.S. Women's Open, stuttered on the back nine before coming home with a three-under 69.
Ko, one of the top players on the Korean circuit, carried her strong domestic form into the $2 million U.S. LPGA event with a roller coaster round of 67 that featured eight birdies.
Yin said an eagle at the par-five fifth had breathed life into a quiet opening to her round.
"That got everything started. I made a few pars in the beginning, but that eagle just fired everything up," she said.
Chun, whose two wins on the LPGA Tour have both been majors (2015 U.S. Open, 2016 Evian Championship), has been runner-up five times this season but said it had been far from a disappointing year.
"Of course with every event you want to win, but you can't win every time. That's kind of like life," she told reporters.
"I know that every shot, every putt throughout the 18 holes can't be perfect, but yet I strive for perfection. Because of that tendency I tend to overthink."
Park, a virtual lock to win Rookie of the Year and who currently leads the Tour's money winning standings, looked set to power clear of the field after her fifth birdie of the day took her to 10-under through 10 holes.
But her momentum stalled with a bogey on 13 where she had to take a drop for an unplayable lie after her hooked second shot at the par-five landed in thick bushes.
"I think after the 13th hole I kind of became antsy and my putting stroke was a bit off," said Park. "After that I missed a lot of short putts, so that was a bit unfortunate for me."
Park missed another birdie opportunity at the par-five final hole and said it was not easy for her as a big hitter to walk away from the longer holes without picking up shots.
"I am a long hitter but doesn't mean that I'm guaranteed a birdie on a par-five hole. I am trying to be more comfortable about that," she added.
Defending champion Carlota Ciganda of Spain got back into the mix with a five-under 67 to stand six shots behind the leader heading into Saturday's third round.
(Reporting by Peter Rutherford; Editing by John O'Brien)
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