McPherson had a 69 Saturday. She and Kerr are at 12-under-par 204, a stroke ahead of South Korea's Jiyai Shin (69).
Hall of Famer Se Ri Pak (72) leads a seven-way tie for fourth at 10-under 206 that includes Ai Miyazato (65), Angela Stanford (69), Suzann Pettersen (72), Helen Alfredsson (71), In-Kyung Kim (69) and Amy Hung (69).
Kerr has 12 career LPGA Tour victories and two this season. McPherson has not won on the LPGA Tour.