Ichinose and Higa both carded six-under rounds of 66 to draw level with overnight leader Oyama.
Ichinose had led by a stroke late on Saturday, but a bogey at the par-three 17th returned her back to a three-way tie.
Oyama returned a second successive 68, dropping her only shot at the par-four 15th.
American Brittany Lincicome and Asako Fujimoto were a shot off the leaders after rounds of 68 and 67 respectively.
Defending champion Stacey Lewis, third in the Rolex Rankings and seen as the favourite this weekend, was on five under after an improved round of 68.
Scotland's Catriona Matthew was the best placed of the European challengers as she shot a second successive 71 to be two under overall.
Matthew birdies three of the opening seven holes to be four under, but bogeys at 14 and 16 dropped her back.