Park used the stellar round to extend her rebound from a rough outing last week, in which she missed the cut of the U.S. Women's Open.
She finished Round 3 at 20-under-par 193, one shot better than Angela Stanford.
Sanford, who came into the day in second place, and remained there with a round of 64 at Grey Silo Golf Course.
Alone in fifth at 15-under was Anna Nordqvist (67).
World No. 1 Inbee Park carded a 68, finishing in a ninth-place tie at minus-13.