By Matthew Geagan, CBS Boston

BOSTON (CBS) — The Bruins’ top line of Patrice Bergeron, David Pastrnak and Brad Marchand was one of the best in hockey during the regular season. They were a force on even strength and even more dangerous on the power play as the Bruins cruised to a 107-point season.

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They were expected to keep their dominance going in Boston’s first-round matchup with the Maple Leafs. But they disappeared again in Monday night’s Game 3 loss in Toronto, with the Leafs’ top line of John Tavares, Mitch Marner and Zach Hyman and top defensive pairing of Jake Muzzin and Nikita Zaitsev limiting their ability to do much of anything.

The Bruins now trail the series 2-1 with Game 4 set for Wednesday night in Toronto. After another night of nothing from his top attack, Bruce Cassidy may have to consider breaking up his once super trio.

Bergeron, Marchand and Pastrnak have tallied three goals so far this series, though two of them came off Bergeron’s stick with the Bruins on the power play. Surprisingly, Bergeron doesn’t have an assist yet in the series. Marchand’s Game 2 tally is the trio’s only 5-on-5 goal in the set.

Pastrnak entered the series with 32 points in his 22 career games against the Leafs, but has no goals and just one assists in the first three games. He managed just three shots in his 22 minutes of ice time in Monday’s 3-2 defeat. He hasn’t been comfortable on the ice, with Toronto taking a physical approach with the 22-year-old, and looks nothing like the 38-goal-scorer that he was during the regular season.

“It’s more on us,” Pastrnak said after Boston’s top line was limited to just seven shots on net in Game 3. “We’re getting a bit frustrated but it’s a long series. We’re going to regroup and focus on the next game.”

“It’s about keeping the puck [in their zone], and once you do have possession it’s about getting it to the net a little bit more,” said Bergeron. “We can’t stay on the outside and we have to find a way to create those chances.”

Marchand is passing up scoring opportunities as he looks to set up teammates, but nothing the Bruins did was very crisp on Monday night. Passes skipped away from their intended targets, allowing Toronto to keep the Bruins out of their zone when they needed to up their attack.

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“They played hard and upped their compete level. Ours wasn’t as good as it needed to be,” admitted Marchand. “But we’re right there, we had opportunity to tie it up late. The difference was special teams so we have to clean it up and go get the next one.”

Boston’s top power play unit of Bergeron, Marchand, Pastrnak, Jake DeBrusk and Torey Krug also looked garbled throughout Monday’s loss. The B’s converted on just one of their three power play opportunities, and that came off the stick of Charlie Coyle on the second PP unit. It cut the deficit to 3-2 with 38 seconds left in the second period, but that was as close as the Bruins would get. Boston received a gift of a power play early in the third when Zaitsev sent the puck into the stands for a delay of game, but they managed just one shot during the two minutes, and it wasn’t even from the top unit. Even Cassidy admitted that his top power play group “was a bit off” on Monday.

The head coach, however, was not ready to throw his first line into the blender. While they are pressing a bit, he knows if they can get over the hump, great things will happen for the Bruins.

“I think they’re trying real hard one-on-one to get there. They’ve got to use each other a little bit better and get an old-fashioned goal where there’s a center lane drive, pucks to the net, second chance. They’re pretty determined guys,” he said. “They’ll find their way.”

Bergeron also admitted that they can’t be going for pretty goals all the time, and need to put the puck in the back of the net any way they can.

“We can be better,” said Bergeron. “That’s what the playoffs are all about, it’s about bringing your best and we’ve got to do that.

“I think it’s tight hockey on both sides. Five-on-five right now, we have to find ways to create more,” he added.

If Cassidy does want to shuffle things up a bit, he could move Pastrnak down to David Krejci’s line and bump Danton Heinen up alongside Bergeron and Marchand. Or he could let his top group play through their current woes, hoping they figure out what is holding them back before it’s too late.

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Chances are we won’t know Cassidy’s plans until the puck drops for Game 4, even if he does scramble things a bit in practice. But changing things up and trying to jump-start his top group has to be a consideration at the moment, especially with the Bruins in a 2-1 series hole.