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.
American Justin Thomas shot his third-straight 67 to take the lead on day three of the Tournament of Champions.