The venerable bromide — it’s not how you start, it’s how you finish — was certainly appropriate Wednesday night because the Rangers fell behind by two goals then roared back with five straight to beat the Bruins, 5-2, at Madison Square Garden for their third straight win.

Brandon Pirri scored twice as the Blueshirts scored the quintet of goals, starting at 9:16 of the second period with a chip shot by Rick Nash, and ending early in the third. Rookie Jimmy Vesey finished a two-on-one rush off a backhanded pass through the crease by Nash, burying the fifth at 3:06 of the third.

“It doesn’t hurt getting depth scoring, for sure,” said Pirri, who scored on the power play and then his fourth of the season, at even strength, just 2:59 apart. “In the greater picture of things, everyone’s got the same system, lines one through four.”

With 27 goals in seven games, the Rangers are averaging a league-leading 3.86 per game. Nine players have 2-4 goals.

“You look at the teams that have success and it’s four lines,” said Derek Stepan, who had two assists. “I hate talking about years in the past, but the year we went to the Cup Final we had four lines. That’s the importance of it. Every team in the past that’s had a lot of success has had depth up front.”

The victim of the flurry was Zane McIntyre, 24, the goaltender who made his first NHL start and was the fourth different netminder in the last four games for the Bruins, who lost both Tuukka Rask and Anton Khudobin to injury. Malcolm Subban lost 5-0 to the Minnesota Wild on Tuesday.

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After sleepwalking and fumbling pucks for the first 20 minutes and trailing 2-0, the Rangers rebounded in the second period with three unanswered goals.

Jesper Fast’s hustle created a 2-2 tie, when he battled in the corner, causing the puck to squirt in front and get inadvertently tapped through a kneeling McIntrye (26 saves) by David Krejci at 16:35. Kevin Hayes was awarded the goal.

The go-ahead score came late in the second period on a power play when Matt Beleskey crunched Girardi, who had returned to the ice after taking a rough hit midway through the period, and was whistled for interference. From the left corner, J.T. Miller found Pirri in the opposite circle and he rifled a shot into the high corner at 19:24.

The Rangers trailed early for the fourth time in seven games. Off the opening faceoff, Nick Holden turned the puck over and Pastrnak skated from the left wall and untouched into the crease. He slid the puck under Henrik Lundqvist (27 saves) at the 10-second mark.

Before the game several players talked about the need to test McIntyre early with shots and screens. The Rangers did neither. The first shot came with 13:11 left, a long softie that was gloved easily by McIntyre.

After Ryan Spooner was called for interference at 7:40, the Rangers did force McIntyre to make four stops. The best opportunity came when Miller cut across the slot, but McIntyre caught his backhander. He later stopped Miller darting in with his right pad.

A turnover by Kevin Klein, who spun near his blue line and flipped a pass to no one in the neutral zone, allowed the Bruins to counter. Czarnik fired toward Lundqvist, who was partially screened by Brady Skjei. The puck hit Lundqvist, bounced behind him and trickled across the goal line for a 2-0 lead at 14:44.

“We were on our heels,” coach Alain Vigneault said. “Our focus was winning the second period, which we did and got a strong start in the third.”