Game 4 is Thursday in Washington. After that, the best-of-seven Eastern Conference series shifts back to Boston for Game 5 on Saturday.
Brian Rolston, Rich Peverley and Daniel Paille also scored for the Bruins.
Alex Ovechkin, Alexander Semin and Brooks Laich got Washington's goals. Laich scored on a breakaway with 6 minutes left to make it 3-3, but Chara came through with 1:53 remaining for second-seeded Boston.
Holtby, who blocked 72 of 74 shots through the first two games, was good but not great Monday, making 25 saves.
The action was more free-flowing than it had been in the series, and perhaps the teams were due. The Bruins ranked second in the NHL during the regular season with 269 goals, and no team had more players with 20 goals than Boston's six. The Capitals have emphasized defense lately, but they still boast talented forwards such as Ovechkin, Semin and Nicklas Backstrom.
Game 1 went to overtime before Boston won 1-0, and Game 2 wasn't decided until the second extra period, when Washington claimed a 2-1 victory. Two games and a whopping total of four goals.
There were four goals in 1½ periods Monday, and a lot more hitting and fighting and penalties. After one prolonged skirmish involving a handful of players, Capitals defenseman Karl Alzner made a fist and rotated it near his eye, as if to say to one of the Bruins, "Why are you being such a crybaby?"
Ovechkin attempted to set the tone right away, flattening defenseman Dennis Seidenberg with a big, roar-inducing hit less than 30 seconds in. About 10 minutes later, Ovechkin was at it again, rattling Rolston, one of the five hits the Russian accumulated in the first period.
Nearly every save made by Holtby especially snatches with his glove drew raucous cheers from the red-clad fans in the stands. He caught a 1-on-1 try by Milan Lucic in the first period, inspiring chants of "Holt-bee! Holt-bee!"
The spectators tried to bother Thomas, mocking him by chanting his name and showing signs that made reference to his decision to skip the trip to visit the president. Thomas, who was born in Michigan and went to the University of Vermont, said at the time he believes the federal government "has grown out of control."
Chara is Boston's best defenseman and it's likely not a coincidence that Washington went ahead 1-0 on a power play with him in the penalty box for roughing.
The Capitals capitalized 29 seconds into the 5-on-4 chance, with Laich corralling the puck, keeping it in the zone and dropping it to Semin, who made it 1-0 at 16:00 of the first period.
Ovechkin was credited with a secondary assist on the goal, the first of the series on a power play. Until then, Washington was 0 for 5, Boston 0 for 7.
The opening 20 minutes ended with a bit of a scrum several feet to Thomas' left. Nothing of the sort seen in the rowdy Flyers-Penguins series, but there was enough improper contact that two players began the second period off the ice: Washington's Backstrom for cross-checking, and Boston's Lucic for roughing.
With some 4-on-4 action, the teams produced two goals in 13 seconds.
Boston tied it at 1, with Peverley scoring off a slap shot from the left circle 35 seconds into the period.
It didn't take long for Ovechkin to respond.
Limited to zero goals and only four shots on net through the first two games, the two-time NHL MVP put Washington ahead 2-1 at 48 seconds. Laich lobbed a pass up ahead, and Ovechkin pounced, beating Seidenberg to the fluttering puck and one-timing a shot under Thomas' left leg.
The lead didn't last long, though. At 9:38 of the second period, Boston tied it, thanks to an unlikely source. Paille who had nine goals all season, and none since Feb. 11 batted down a rebound with his stick, then nimbly got around Holtby and poked home the puck to make it 2-all. Rolston put Boston ahead 62 seconds into the third period, before Laich temporarily evened the score.
NOTES: Ovechkin's goal allowed him to break a tie with coach Dale Hunter and move into second in Capitals history with 26 in the playoffs. Only Peter Bondra, with 30, scored more postseason goals for Washington. ... Thomas turned 38 on Sunday. Holtby is 22. ... The Bruins have won six consecutive road games, dating to the regular season. Schedule
• Boston 4, Washington 3
Bruins lead series 2-1
• NY Rangers 1, Ottowa 0 Rangers lead series 2-1
• St. Louis 4, San Jose 3
Blues lead series 2-1
• Florida at New Jersey, 5 p.m., NHLN
Series tied 1-1
• Nashville at Detroit,5:30 p.m., NBCSN
Predators lead series 2-1
• Phoenix at Chicago, 7 p.m., CNBC
Series tied 1-1