BOSTON — The Bruins’ losing streak has reached three.

Boston fell to the Ottawa Senators, 6-2, on Sunday night on its second game of a back-to-back. It marks the first time this season the Bruins have lost three straight games at TD Garden.

A sleepy start for the Bruins led to the Senators scoring three goals on nine shots in the first period.

It took the Senators 90 seconds to get on the board with a Drake Batherson snipe that beat Joonas Korpisalo to give Ottawa a 1-0 lead.

Ottawa took a 2-0 lead after a Batherson kick pass set up Claude Giroux for a goal, though it appeared Batherson got away with a hold on Andrew Peeke’s stick behind the net.

Fabian Zetterlund made it 3-0 three minutes later with a power-play goal after he was left alone to fire a shot by Korpisalo’s shoulder.

The Bruins had a 5-on-3 after Tim Stützle was called for hooking and Artem Zub was whistled for tripping 24 seconds later. And it was David Pastrnak who hammered home a one-timer by Linus Ullmark to make it 3-1 before the first period came to a close.

The second period didn’t start well for the Bruins, who allowed a Stützle goal and a Dylan Cozens goal 19 seconds apart to make it 5-1.

Stützle’s goal forced Marco Sturm to pull Korpisalo from Jeremy Swayman — who made his fourth straight start on Saturday against the Vancouver Canucks.

Things looked like they went from bad to worse for Boston when Marat Khusnutdinov laid a big hit behind Ottawa’s net and immediately went to the dressing room in visible pain.

But the top-line forward returned later in the period.

Charlie McAvoy scored the second goal of the game for the Bruins — and the defenseman’s first of the season on his 28th birthday — to make it a 5-2 game.

The Senators answered three minutes later on Zetterlund’s second power-play goal of the game for the 6-2 lead.

Despite the four-goal deficit, the Bruins weren’t going to mail in the game. Mark Kastelic dropped the gloves with Kurtis MacDermid in a bout that saw both players throw several punches.

The two teams got into it with 1:01 left in the game with Pastrnak dropping the gloves.

The Bruins were unable to carry momentum over from the fights and fell to the Senators for the third time this season.

The Bruins return to action on Tuesday when they host the Montreal Canadiens before the NHL’s holiday break.