Khang, who broke through for a first career win after 190 starts in Canada on Sunday, carded seven birdies in her third-straight six-under par 66, five of them on the tough back nine at Columbia Edgewater in Portland Oregon.
Jang, a 20-year-old South Korean whose rookie LPGA season has been disrupted by injury, had 11 birdies in a remarkable 10-under par 62 that put her on 17-under 199.
She was joined by Monday qualifier Chanettee, a 19-year-old rookie from Thailand who had eight birdies in her seven-under par 65.
Khang said this week that she felt some residual fatigue after her playoff victory over South Korean star Ko Jin-young in Canada, but that was balanced by an increase in confidence "knowing I can win out here.
"It's not just if it was going to happen or when but it has happened, so it's definitely a great feeling to have under the belt."
After a pair of birdies and a bogey on the front nine, Khang caught fire coming in, rolling in three straight birdies at 11, 12 and 13 and draining another long birdie at the 15th to share the lead.
She rattled in one more at 18 to put herself in front.
"I did know that I was five-under on the day and I kind of needed that one to do a six-under," she said. "In my head I was like, all right, let's try to be consistent out there and try to get all three rounds at six-under. Proud of that."
Jang said she had no indication warming up that her big round was coming, but after a rainy start to the week she was happy to play in warm sunshine in a pairing with long-admired compatriot Kim Hyo-joo and, most importantly, without the back pain that has plagued her much of the year.
The birdies piled up quickly - four in her first four holes and another pair at the sixth and seventh before a bogey at the ninth.
Focused on the present
Five more birdies coming in, including a tricky downhiller at 18, pushed her to the top of the leaderboard before Khang slipped past.
"Honestly I wasn't really tracking my results, but it was going good and I thought it was great, but I wasn't really counting birdies," she said. "I was more focusing on the present."
Jang said "kind of everything" went right for her, from strong iron play that left her some tap-in birdies to long putts to a chip-in.
Chanettee said coming through Monday qualifying had given her a much-needed confidence boost, as she seeks to finish strong in a season in which she's missed nine cuts.
With the course still soft from rain earlier this week, Sunday promised to be a shoot-out.
Spain's Carlota Ciganda, seeking a third LPGA victory, was alone in fourth on 201, while four players shared fifth on 202.
That group included China's Lin Xiyu and Yin Ruoning - who claimed her first major title at the Women's PGA Championship in June - along with Thailand's Atthaya Thitikul and American Gina Kim.
It was a frustrating day for France's Perrine Delacour, leader after each of the first two rounds who carded a three-over 75 to fall seven strokes adrift.
She was in a group of seven on 205 that also included defending champion Andrea Lee, world number two Nelly Korda and South Korean Lee Mi-hyang, who had nine birdies in the space of 10 holes from the fourth through the 13th holes but had four bogeys the rest of the way.
