Gillman, the 16-year-old Austin, Texas, player who has committed to Alabama, had the best three-day total in the history of the girls' division. She broke the mark of 203 set by Vicki Goetze in 1990 and matched by Ariya Jutanugarn in 2011.
Gillman rebounded from a double bogey on the par-4 ninth with three birdies on her final four holes.
''I didn't hit many good approach shots on the front nine today, and just made a few more mistakes overall,'' Gillman said. ''But I wanted to have a bigger lead tomorrow, so I tried to make as many birdies as I possibly could.''
''I think both girls (Wagner and O'Sullivan) will play really well tomorrow, so I need to play my best,'' Gillman said. ''I need to go out and play my game, have fun, and not get caught up in my score.''
Gordon Neale of Trabuco Canyon, California, had a two-stroke lead in the boys' division when third-round play was suspended because of thunderstorms.
Neale, the 17-year-old Trabuco Canyon, California, player who has committed to SMU, was 8 under overall through 11 holes when play was stopped. Sam Burns of Shreveport, Louisiana, was second. He also completed 11 holes.
The boys' winner will get a spot next year in the PGA Tour's Texas Open and the girls' champion will qualify for the Swinging Skirts LPGA Classic in California.