He posted a 19-under 194 total, while fellow Australian Jason Day and American Rickie Fowler both shot 70 to share second place on 14-under, with Englishman Justin Rose (66) alone on 12-under.
Leishman is taking nothing for granted, after surrendering a two-shot lead two weeks ago and fading to third at the Dell Technologies Championship.
"You need to have it for four days. There’s no point having it for three days," he told PGATour.com.
"To have a lead is really nice but I showed (two weeks ago) a lot can happen on the back nine."
The Conway Farms course was not quite the pushover on Saturday it had been over the first two rounds.
"It got pretty firm out there. I'm really happy to make a couple of birdies coming in and put a little space between me and the other guys," Leishman said after a deft chip at the par-five 18th for a closing birdie.
"Sixty-eight was a really good score out there today I thought."
The BMW Championship is the penultimate event of the PGA Tour season.
Only the top 30 players in the FedExCup points standings will advance to next week's Tour Championship in Atlanta, where a $10 million bonus will be awarded to the season-long champion.
Leishman is projected to jump to fourth in the standings if he wins on Sunday, behind Americans Justin Thomas, Jordan Spieth and Dustin Johnson.
He has won only twice in 222 starts on the PGA Tour, a record that is baffling to his fellow pros, who to a man speak glowingly of his game.
Leishman, 33, is used to hearing spectators saying "who's that bloke," even in his homeland.
"I guess I just fly under the radar a lot," said Leishman, a playoff loser at the 2015 British Open won by Zach Johnson at St. Andrews.
"I need to contend more, need to win more trophies."
Fowler started promisingly by sinking a 25-foot eagle putt from the fringe at the par-five first, but he struggled to buy a putt thereafter, his only birdie a near tap-in at the par-five 14th.
"The biggest thing for me was not seeing anything go in," he said. "The greens were getting pretty beat up, with spike marks. It started to have a lot more effect on the ball rolling."
Day also had a cold putter, the 2015 champion recording just two birdies.
(Reporting by Andrew Both in Cary, North Carolina, editing by Gene Cherry)
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