Simpson, who came into the day with a four-shot shot lead, maintained that advantage after Saturday's round, ending at 19-under 194 in a quest for his first PGA Tour win since the 2012 U.S. Open.
Should he hang on through the final round, Simpson would get his fourth career PGA triumph.
Chesson Hadley, who also logged 67 at TPC Summerlin, had sole possession of second place at 15-under.
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