The Islanders managed to stay tied with the league-leading Canadiens through two rather ugly periods and managed to tie it up again late in the third after giving up a fairly late goal.

But there was no answer for Shea Weber’s booming slap shot. The Canadiens defenseman unleashed a ripper on the power play that blasted past Thomas Greiss with 2:57 to go, the final tally of a 3-2 Islanders loss that felt like the Isles’ poorest performance in this young season last night at Barclays Center.

Much of that was due to the 6-0-1 Canadiens, who pressed the Islanders into turnovers and missed passes through the majority of the first 45 minutes. Only Greiss, who finished with 26 saves, kept the Isles afloat after Paul Byron put the Habs on top 3:10 into the second.

“We just need to be smarter,” said Dennis Seidenberg, whose one-timer beat Al Montoya through Anders Lee’s screen at 14:16 of the third to tie the game 2-2. “They got on us quick and you have to make better decisions. You move your feet and you force them to respect your speed and then you can create some space. We have to do a little more of that.”

John Tavares scored on the Isles’ lone power play at 15:17 of the second, helping the Islanders get to the third at 1-1 after the dreadful, turnover-filled first two periods. Byron’s goal came after Mathew Barzal, playing in his second NHL game, turned the puck over in center ice and Montreal went to work.

The Canadiens’ second goal, a rebound stuff-in by Philip Danault after the Canadiens won the puck off a harmless dump-in, restored their one-goal lead with 8:39 left in the third.

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Jack Capuano shuffled his lines just a bit prior, putting rookie Anthony Beauvillier with Tavares and Josh Bailey and putting Andrew Ladd with Casey Cizikas and Cal Clutterbuck, among other changes.

An old line that was reformed in that third period helped get the Islanders even. Lee and Ryan Strome, out with Brock Nelson, worked for a puck behind the Montreal net. Strome fed Calvin de Haan, who sent a pass across to Seidenberg for a one-timer and another tie game, Seidenberg’s second of the season.

Ladd helped create Tavares’ goal, tying up Alexei Emelin after a rebound, but did not receive an assist and is stuck on one point through seven games.

“We just have to find something, mix it up,” Ladd said. “I’d love to be more productive through seven games but I also know the kind of player I can be and what I need to do. I’m not too worried about it.”

It was another turnover that led to the Canadiens’ late power play and Weber’s big blast. Nick Leddy hacked Brendan Gallagher after the Habs won another puck deep with 3:50 to play; Weber settled into a spot atop the right faceoff circle and Greiss had no chance.

“We definitely didn’t play the way we wanted to tonight,” Leddy said. “We’ve got another one tomorrow (at the Penguins) so we need to get right back to it.”