Hertl gave San Jose a 1-0 lead at 6:10 of the first period on a give-and-go with Rudolfs Balcers when he made a quick move from the side of the net.
Dahlen made it 2-0 with a power-goal at 17:17.
"Great screens there," Dahlen said of his goals. "We had a good breakout and got in the zone quick, and got them through. It was nice."
Forsling cut San Jose's lead to 2-1 at 19:06 when he one-timed Sam Reinhart's pass from behind the net.
Nieto made it 3-1 at 1:31 of the second period when he one-timed Nick Bonino's centering pass.
"That's a great hockey team and [Reimer] played a great game," Dahlen said. "This was the fastest game I've seen in my career."
NOTES: Panthers forward Joe Thornton was in the starting lineup and played 11:41 with two shots on goal in his first game against the Sharks after playing 15 seasons for them (2005-20). It was his 23rd game of the season, his second since Jan. 8. Thornton played 10 games against the Sharks while with the Boston Bruins from 1997-2005. His span of 18 years, 84 days between games against the Sharks is the longest in NHL history for a player against an opponent. ... Sharks defenseman Mario Ferraro left in the first period after being hit in the face by a puck. Coach Bob Boughner said Ferraro was being evaluated by a dentist. ... Florida leads the NHL with three third-period multigoal comeback wins (against the Pittsburgh Penguins on Oct. 14 and Washington Capitals on Nov. 30). ... The Panthers are 23-3-0 at home to equal the 26-game home wins record held by the Sharks (23-1-2, 2008-09), Detroit Red Wings (23-2-1, 2011-12), and Boston Bruins BOS (23-2-1 tie, 1970-71). Florida is 36-5-0 in its past 41 home games. It does not play at home again until Feb. 22.