Starting the day with a six-stroke lead after shooting 69 on Friday, four bogeys and a double bogey on the 16th hole saw the chasing group cut the lead in half, though Chun recovered well to close out back-to-back pairs.
Chasing her third major career – and first in six years – Chun said she was “so proud” of the finish, which kept her a shortened, albeit significant, lead at the top.
“It was a little hard there,” she told reporters.
“I am so excited and already looking forward to an exciting final round. If it’s too easy, then I feel like it’s boring, so I just want to enjoy another tomorrow.
“No matter where I am, I’m just ready to make another good day or another good round if I can. I think this is another process, so just keep going,” Chun added.
After finishing the first round with a stunning 10 shots from Chun, Thompson landed seven hits on the leader over the next two days, helped by a great five-to-67 play on Friday.
An 11-time LPGA Tour winner and major Kraft Nabisco Championship champion in 2014, the 27-year-old is determined not to even look at the leaderboard, leaving that responsibility in the hands of her caddy, Will.
“Honestly, I tried not to watch the leaderboard anymore at all,” Thompson told reporters.
“I’m sure he [caddie] follow him. If I have to go for an injection, he will let me know. I’m just trying to focus on my game, on my emotions.
“That’s all I can control, and whatever the outcome, it’s there.”
Australia’s Hannah Green is four strokes behind the leader in the top four, while Thailand’s Attaya Titicul, Canada’s Brooke Henderson and American duo Jennifer Chang and Jennifer Cupcho are further behind in the top three.
The first group is scheduled to start at 8:29 AM ET Sunday, with Chun, Choi and Thompson among the last to start their final rounds around 10:30 AM ET.