Inkster's 54-hole total is 6-under-par 210 after Saturday's 3-under 69. She will have a two-shot lead over Jeong Jang at the outset of Sunday's fourth and final round.
Only Inkster and Angela Stanford (69) broke 70 for the day.
"I played very well," Inkster said. "I'm driving the ball well and I'm putting good. I've been really struggling with my putting the last month and played really horrible, so it's good to see that come around."
Second-round co-leader Dianna D'Alessio struggled to a 4-over 76.
Two-time winner Annika Sorenstam (74) is tied for sixth at 2-under 214.