Tseng, the current world No. 1 who is going for her 14th career title and second this year, ended the day deadlocked with former No. 1 Miyazato at 14-under-par 202 after 54 holes.
Miyazato is seeking her eighth win and first since last year at the Evian Masters.
World No. 2 Na Yeon Choi, meanwhile, carded her own 67 and finished three rounds one stroke off the lead at minus-13.
I.K. Kim held sole possession of fourth place at minus-11 following a round of 3-under 69.
Inbee Park also registered a 69, ending the round one shot further back at 10-under 206.