Miyazato, the 2010 winner, finished three rounds at 14-under-par 202. She will go for her eighth LPGA Tour title on Sunday.
However, she will have plenty of solid competition.
Women's world No. 1 and defending champion Yani Tseng also carded a 65, while playing alongside Miyazato. Tseng moved into second place at minus-13.
Former world No. 1 Jiyai Shin (68) and World Golf Hall of Famer Karrie Webb (71) are tied for third at 12-under-par 204. Webb had a three-stroke lead after the second round, which was completed earlier Saturday due to a pair of weather delays on Friday.
Amy Yang posted a four-under 68 on the Pattaya Old Course at Siam Country Club and is alone in fifth at minus-11, while world No. 2 and 2007 champion Suzann Pettersen shot three-under 69, but dipped into sixth place at 10-under 206.
Miyazato opened with a birdie on the first, but she gave that stroke back when she tripped to a bogey on the third. She started to make her move up the leaderboard with a birdie at the fifth.
The 26-year-old, who led by one after the first round, birdied the eighth to move to nine-under. She still trailed Webb by three at that point, but quickly erased that deficit early on the back nine.
Miyazato converted birdie efforts on 10 and 11 to gain a share of the lead as Webb faltered to a bogey on the ninth. Webb bounced back with birdies on 10 and 12 to briefly grab a one-stroke lead at 13-under.
However, Miyazato joined her there with back-to-back birdies at 13 and 14. Miyazato made it three in a row with a birdie on No. 15 to grab the outright lead for the first time in the round.
Miyazato, who reached the top of the women's world rankings three times during the summer of 2010 and spent a total of 12 weeks as the No. 1 player, closed with three straight pars to grab the 54-hole lead.
"It was a really, really long day for me," said Miyazato, who played 23 holes on Saturday. "I've been up since 4 a.m. this morning, but the temperature dropped down today so that helped a lot too. So that's why I think I could control myself out there. I kept a really strong focus. It was a really long day out there, but I felt really good."
Webb parred four holes in a row from the 13th before stumbling to a bogey on the 17th. She closed with a par to end the round two back.
"I'll take one-under. I really didn't swing it well and it progressively got worse as it went on so I made some really good pars out there today when I really was struggling with my swing," admitted Webb. "I'm glad that I haven't played my way out of it, I'm only two shots back."
Tseng, who has had a stranglehold atop the world rankings since taking over the top spot from Shin last February, started a bogey-free round with a birdie on the first.
She carded consecutive birdies from the fifth. After a pair of pars, Tseng carded three birdies in a five-hole span from the ninth to join Miyazato and Webb atop the leaderboard.
Tseng ran off four pars in succession from the 14th. She closed with a birdie at the last to get within one of Miyazato, who she will play with again in the final round.
"I hit so many great shots today and I just feel like my game is there. I'm pretty confident and just will try to do it again tomorrow," Tseng stated.
NOTES: The top four players on the leaderboard were among the 14 players that finished their second rounds earlier Saturday...Last year's Rookie of the Year Hee Kyung Seo carded a 10 on the par-five 18th, which was her ninth in round three.