The 23-year-old hit three consecutive birdies as he secured a two-shot lead over Japan's Hideki Matsuyama.
Americans William McGirt, Jimmy Walker and Ryan Moore are tied in third place with 14 under in Hawaii.
World number one Jason Day is in 13th place after a three-under 70, while Scotland's Russell Knox is seven shots off the lead with one round remaining.
"I left a lot our there. That is always a good thing because I can just go work on it and improve," Thomas said.

The 25-year-old will join the Women's Super League One club following this summer's European Championships, for which Belgium have qualified.
The former FC Twente forward has scored four goals in 26 international games.
"Yana was brought to my attention after recently playing against Scotland," head coach Willie Kirk told the club website. "Her style of play suits us and she is a big personality."
American Justin Thomas shot his third-straight 67 to take the lead on day three of the Tournament of Champions.
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Bristol City Women have signed Belgium striker Yana Daniels from Anderlecht.