The 22-year-old Masson made three birdies in the front nine and three on the back to hold off a late charge from Yani Tseng.
The top-ranked Taiwenese player closed within a shot at one stage after picking up five strokes in as many holes, including an eagle. She finished with 66.
Catriona Matthew, of Scotland, leads the home challenge, but she is six shots off the pace after shooting 68. Inbee Park, of South Korea, is also at 207 for a share of third place.
Masson, who is at 15-under 201, will play the final round today with Tseng, the 22-year-old prodigy who already has won four majors.
"It should be really enjoyable. Yani seems very nice and I'm sure I'll learn a lot playing alongside her," Masson said.
For Tseng it also will be a new experience.
"I have never seen her play," she said.
First-round leader Meena Lee, tied for second overnight, fell out of contention with an 80 to sit at 2-under.
EUROPEAN TOUR: In Killarney, Ireland, David Howell shot 7-under-par 64 to take a share of the Irish Open lead with Simon Dyson and Richard Green.
The 36-year-old Howell ended up sharing the lead when Dyson birdied the 17th hole and Green birdied the last. Both finished with rounds of 67 to join Howell at 11-under 202.
Homegrown stars Rory McIlroy and Graeme McDowell fell out of contention with 72s.
McIlroy had a terrible start, double-bogeying the first and eighth holes. He also bogeyed the par-3 third as his wind-blown tee shot sailed left into a stream. He finished eight shots behind the leaders, as did McDowell.
Former Augusta State golfer Oliver Wilson had 74 and is one shot in front of McIlroy and McDowell.
Disappointment was the order of the day for the Irish hosts. Only two of the seven remaining Irish players broke par.
The 22-year-old McIlroy, who won the U.S. Open in record fashion, conceded he forced shots in hopes of entertaining the home fans.
"At this tournament, you're trying so hard to play well for not just yourself, but for everyone else," McIlroy said. "Sometimes you can just find yourself trying a little too hard and pushing a bit too much."
NATIONWIDE TOUR: In Sandy, Utah, North Augusta's Scott Brown and Aiken native Matt Hendrix moved into the top 15 after three rounds of the Utah Championship.
Brown and Hendrix both shot 3-under-par 68s. Brown moved up from a tie for 25th to a tie for 12th at 204. He's seven shots off the lead shared by J.J. Killen and Jeff Gove.
Hendrix went from a tie for 34th to a tie for 15th at 205. He's eight shots back.