The Norman, OK teenager shot a 4-under 68 to take a one-shot lead over UCLA freshman Patty Tavatanakit of Thailand.
Maria Fassi (Mexico), Yvie Chaucheprat (France), and first- and second-round leader Lily May Humphreys (England) are tied for third, three shots behind Son.
Son started the tournament with an even-par 72, and then got into contention with a 67 on Wednesday. Her momentum continued with a bogey-free 68 in round three, with three birdies in a row from 14-16 giving her the lead. In 54 holes, Son has made only two bogeys.
Tavatanakit matched Son's 68, riding a front-nine 32 to put herself in a good position to win. She is currently 8 under par, one shot back.
The 18-year-old Chaucheprat had the round of the day, a 5-under 67, to reach 6 under for the tournament. Chaucheprat has improved each round of the tournament, with rounds of 73-70-67, and has made 13 birdies in the last two rounds.
Fassi, a junior at the University of Arkansas, overcame an early double bogey to post a solid 70.
Humphreys, the 15-year-old who won both the English Women's Amateur and the British Girls' Championship this year, held the lead after each of the first two rounds with two straight 68s. Her good play continued through the front nine on Thursday, with a 1-under 35 getting her to 9 under for the tournament. But a bogey at the par-four 13th and a double bogey at the par-five 16th knocked her out of the lead and left her three shots back.
Related: Women's Dixie: English champ Humphreys Leads By 3
Aline Krauter, a Stanford commit from Germany who lives in Wesley Chapel, had an up-and-down round of 71 that leaves her in solo sixth, five shots behind Son.
So is winning is becoming a habit for Son, she should be formidable in her quest to repeat in Friday's final round. But with the quality of the chase group, she can't expect anyone to fade this time around.