Gulbis charged from three stokes back, notching three straight birdies from the eighth hole to gain a share of the lead at Tiburon Golf Club.
She finished 54 holes at 11-under-par 205, along with Piller and Pornanong Phatlum, each of whom carded a 5-under 67.
Overnight leader Sandra Gal stumbled to a 2-over 74, falling into a fourth-place tie at minus-9 with Stacy Lewis -- who completed a course-record 9-under 63 -- Shanshan Feng and Lexi Thompson (both 67s).
Sole possession of eighth place went to Michelle Wie (66) at 8-under-par 208.
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