Yubol, the second-round leader, had a 68 to match Schubert at 16-under 200 with a round left on the tree-lined Columbia Edgewater Country Club layout.
The leaders are winless on the LPGA Tour, with the 30-year-old Schubert back on the big tour after finishing 10th in the Epson Tour standings last year. The American — the 2017 US Women's Amateur winner — is ranked 295th in the world and is 125th in the tour's points race.
"I love the Epson Tour. It's great. I loved playing out there," Schubert said.
"I think it was a great boost for my confidence. Competition is great and you get the sense of winning."
The 24-year-old Yubol is ranked 112th in the world and is 28th in the tour's season standings. She finished second this year at Mayakoba and the ShopRite LPGA.
Fellow Thai player Jeeno Thitikul is third, two strokes back after a 67, to take final spot in the last group on Sunday (Monday AEST). She's chasing her 10th LPGA Tour victory.
"We're playing golf because we love to be in contention," Thitikul said.
"We love to chasing the trophy."
South Korea's Narin An shot 66 to get to 13 under.
Robyn Choi (71) is the leading Australian at eight under, while Grace Kim (74) is a further shot adrift.
Schubert rebounded from a bogey on the par-4 14th with birdies on the par-3 16th and the par-4 18th. Playing partner Yubol birdied the first two holes on the back nine to get to 17 under, then had six pars and a bogey — on the par-4 15th — on the final seven.
"I can't wait to play with her again," said Schubert, who played in college at Auburn and Texas.
"We had a great day today. She's fun to play with."
With AAP.
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