Thornton missed the final three games of the regular season and Games 1 and 2 in Edmonton because of a left-knee injury. He has been skating since the series began and has said on a daily basis that his knee was feeling a bit better.
Thornton, 37, returned to his usual game-day routine Sunday, skating for about 10 minutes prior to his teammates at the optional morning skate.
"After yesterday with the day off, I wanted to do something light and just kind of feel a little bit fresher today," he said afterward. "So hopefully it responds good and I'll be able to play tonight."
Thornton was the first player onto the ice behind Sharks starting goalie Martin Jones for warmups.
The best-of-7 series is tied 1-1.
Game 3 is the 157th NHL playoff game of Thornton's career.
