The lineup was changed significantly to address the physical challenges of competing in a playoff game for the second night in a row, and for the third time in four nights.

But the Bruins now trail their best-of-seven second-round series with the Lightning, 2-1, because of how they mishandled the mental challenges in Game 3 on Wednesday night in a 7-1 loss that was their most lopsided postseason defeat since 1992. And while they may be better rested and perhaps healthier for their next scheduled game, the physical and mental challenges will only be greater, because they’ve let their opponent get on a roll.

“As a whole, let’s face it: They were better than us in every facet of the game,” Bruins Coach Bruce Cassidy said. “We’ve got our work cut out for us.”

Cassidy put a lot of work into Game 3 in a short time, but none of it paid off.

He changed his lineup to an 11-forward, 7-defenseman group to add some size (John Moore, Jeremy Lauzon) to a previously smaller back end that had showed signs of wearing down against a tight schedule and a bigger, more physical Lightning team than the super-skilled roster of years past. Cassidy challenged what he calls “that second layer of our group, that have been in the league and who could be the future of the Boston Bruins … to step up and pull the veteran core along.”

Nothing worked.

The Bruins, without hard-nosed forward Sean Kuraly (unfit to play) or small but active defenseman Connor Clifton (apparently a coach’s decision), became shockingly fragile once breaks went against them in the first period, and there were almost no signs of recovery.

Jaroslav Halak, who started for the sixth straight game and second time in as many nights but didn’t finish, couldn’t do much when Ondrej Palat scored during a power play that Cassidy didn’t think was earned (Nick Ritchie’s slash behind the play, the coach said, “happens 100 times a game”), on a shot that deflected off Zdeno Chara’s stick.

Lauzon couldn’t do much 15 seconds later to avoid linesman Devin Berg, who accidentally blocked the rookie out of a race with Yanni Gourde, giving Gourde a clear path to the net for a goal that made it 2-0.

The Bruins didn’t act like they still had 47 minutes left to make it a game, though. They finished the first period with only eight shots on Andrei Vasilevskiy, who effectively got a night off after seeing only 25 shots in Game 2. Their one sign of life came at 4:56 of the second period, when the most dependable segment of their game throughout the playoffs – the power play – produced a goal from Brad Marchand that cut the Lightning lead to 3-1.

But the Bruins, who had killed all 10 penalties over their previous four games, let the Lightning – 0 for 16 on the manpower advantage before Palat scored – net a third power-play goal (Alex Killorn) at 8:18, and “you’re just starting to build your game for Game 4” at that point, Cassidy said.

Even that didn’t go well: Halak (12 saves) was pulled at 11:18 of the second to start preparing for Game 4, but the Bruins did little to protect 23-year-old Dan Vladar in his NHL debut: Vladar (12 saves) had to face three breakaways in less than half a game, and the Lightning got goals on two of them.

“He was obviously put in a tough position,” Chara said.

That’s where the Bruins find themselves, and it’s largely of their own doing. They let the Lightning off the hook in Game 2, a 4-3 overtime loss in which they didn’t protect leads of 1-0 or 2-1 and didn’t take advantage of the same type of breaks the Lightning seized in Game 3. The Lightning didn’t have big-minutes defenseman Ryan McDonagh, they were upset that a slow whistle let the Bruins take a 1-0 lead, and they had a goal taken away by a successful video review.

But they recovered from all that to win Game 2, and clearly rode that momentum throughout Game 3. The Bruins, meanwhile, “didn’t respond,” Cassidy said. “The disappointing part … is that we weren’t able to get ourselves back in the game by killing any further penalties, or creating offense, or having a pushback.”