American Johnston reels in Herbert at Australian Open
In the women's event, South Korean veteran Shin Ji-yai surged up the leaderboard with a 67 to be two shots clear of Australian world number six Hannah Green (68).
South African two-time defending champion Ashleigh Buhai (67) is two strokes further back, setting up a grandstand finale between three major winners.
The mixed Australian Open involves men and women teeing off in alternating groups on the same courses.
Australia's Herbert took a four stroke advantage into the weekend, but a bogey at the fourth saw him make the turn in par.
In contrast, his playing partner Johnston sank five birdies on the front nine to race into a two stoke lead.
But Johnston, who came through the DP World Tour qualifying school this year, was derailed by a double bogey on the 14th, allowing Herbert back in.
They lead by two from a group of five, including China's Ding Wenyi and Chilean defending champion Joaquin Niemann, who thrust himself into contention with a sizzling 64 -- the best round of the day.
"I'm just going to try and play my own game (tomorrow), hit fairways and hit greens and just keep myself in it," said the Montana-born Johnston.
"Whatever happens, happens, it's been a great learning experience. It's my second week on the DP World Tour and I'm in contention, so that's great."
Herbert said he was frustrated with the way he handled a wet day at Kingston Heath in Melbourne's famous sandbelt region.
'A big chance'
But he is confident victory remains well within his grasp.
"Frustrating day but I haven't gone backwards, I still lead the tournament and I'm massively in it for tomorrow, I've got a big chance," he said.
"It can be very easy to want it too much, try too much and I've got to just make sure I don't do that tomorrow and be patient and take every shot as it comes."
After imploding on the back nine on Friday, 2022 British Open champion Cameron Smith was unable to get back in the hunt and is 12 behind the leaders.
South Korean amateur Yang Hyo-jin, just 17, was the overnight women's leader by a shot.
But with Shin, Green, and Buhai breathing down her neck, she folded under the pressure, dropping three shots in her opening four holes and never recovering to end the day eight adrift.
Green, a three-time winner on the LPGA Tour this year, was one clear heading to the 17th but fluffed a bunker shot and dropped two strokes before a birdie at the last.
Two-time major winner Shin, who won the tournament in 2013, was steady all day, firing seven birdies, including at the 17th and 18th.