BOSTON — Bobby Ryan took a punch in overtime, then delivered one of his own to win it.
"I always say that you make your money during the season, but you make your reputation in the playoffs, and right now he’s showing everybody that he’s a gamer," Senators coach Guy Boucher said of Ryan.
The penalty on Nash drew the ire of the sellout TD Garden crowd of 17,565, as well as Bruins interim coach Bruce Cassidy.
"Demoralizing and disappointing," Cassidy said. "There’s probably a lot more words, but they called it. Once they called it, it’s our job to kill."
Mike Hoffman scored two goals and Derick Brassard added another for the Senators. Craig Anderson made 17 saves.
"It feels incredible and we took home-ice back and that’s what’s important," Ryan said.
Noel Acciari, David Backes and David Pastrnak scored for the Bruins while Tuukka Rask had 28 stops.
It is the second straight game in the series that has gone to overtime, with Dion Phaneuf scoring 1:59 in Saturday’s extra period to send the Sens to a 4-3 victory in Game 2.
"That’s the NHL. In (a) matter of a few minutes, the tide changes," Boucher said.
Game 4 is Wednesday in Boston, where Ottawa has won four straight.
Hoffman polished off a breakaway after a lengthy saucer pass from Erik Karlsson to put Ottawa up 1-0 at 7:40 of the opening period.
Brassard struck 25 seconds later, hammering home a tip from Ryan after Viktor Stalberg’s pass from behind the net, making it 2-0.
The Bruins managed three shots on goal in the first and just one in the final 16:18 of the period, and neither side got off a shot in the last 9:47.
Hoffman’s power-play snipe 3:42 into the second put the Sens ahead 3-0.
For the second straight game, the Bruins erupted for three goals in the second to bring an anxious crowd to life.
Acciari, making his playoff debut, scored on a redirect at 6:05 to cut the deficit to 3-1. Backes stole the puck from Ryan and netted another goal 42 seconds later to make it 3-2.
Pastrnak tied it with his first career playoff goal on a wrister seconds after the Bruins’ 5-on-3 advantage ended at 13:51.
Anderson stopped all nine shots Boston sent his way in the third and Rask brushed aside seven to send the game to overtime.
"We showed good character coming back, but we’re not gonna win many games getting down 3-0," Bruins left-winger Brad Marchand said.
David Krejci returned for the Bruins after missing the first two games of the series with an upper-body injury. Colin Miller skated during warmups but missed his second straight game after suffering a lower-body injury in Game 1.
Notes: Sens D Ben Harpur made his playoff debut, replacing Mark Borowiecki after he suffered a lower-body injury in Game 2. … Karlsson was not on the ice for Monday’s morning skate. "He never needs to practice," Boucher said. … Boston D Tommy Cross also made his playoff debut.