PITTSBURGH — Three times in a row against the Penguins, the Islanders have gone into the third period needing a comeback.

And three times in a row, they’ve delivered.

The Islanders completed a season sweep of the Penguins on Thursday with a 4-3 overtime victory following two third-period goals that seemingly would have come from nowhere, if it were not the third act of the same movie we saw on Feb. 17 at UBS Arena and Feb. 20 here in PPG Paints Arena.

Brock Nelson provided the overtime goal to put an exclamation mark on another stunning comeback against the Penguins, as well as an end-to-end period of three-on-three hockey.

This was a testament to the Islanders’ resiliency — and a fairly baffling set of results for a veteran Penguins team — which has kept the Isles’ hopes of staying in front of Pittsburgh for the first wild-card spot very much alive.

With 76 points in 67 games, the Islanders hold at least an ostensible advantage over the Penguins, who have 73 points in 64 games.

If Pittsburgh is going to both make the playoffs and avoid the Bruins in the first round, it will require making good on their games in hand.

As for the Islanders, they saw this game turn on a dime after 40 minutes of disastrous hockey, in which breaking out of the zone looked an impossible task and defensive-zone structure seemed an improbable concept.

It was who else but Hudson Fasching who got the comeback started, diving on a Casey Cizikas feed to the crease with 5:29 left in the game to cut Pittsburgh’s lead to 3-2.

Until then, the Islanders had played a middling third period, failing to convert on two power-play chances and possessing the puck without giving Tristan Jarry much work.

But that goal gave the Islanders the belief they needed.

Anders Lee tied the game with a six-on-five goal with 1:15 to go, deflecting the puck through Jarry’s five-hole to send it to overtime.

Once there, it was just a matter of the finishing move.

Outside of Lee’s power-play goal 5:23 into the match, the Islanders spent just about the entire first 20 minutes on the back foot.

Twice, that resulted in Penguins goals.

Jake Guentzel tipped in Marcus Pettersson’s point shot at 11:07, then Jason Zucker got free in the slot to one-time a feed from Evgeni Malkin in at 18:24.

During a second period, when the Islanders’ shot total was nearly doubled, the only goal the Isles allowed was on a Josh Archibald tip-in at 10:49.

Still, that belied a failure to generate anything of note offensively — a trend that went through the first 40 minutes.

Just over two weeks ago in this same building, the Islanders received news that Mathew Barzal would be out on a week-to-week basis with a suspected knee injury.

Since then, they are a logic-defying 6-1-1 with a flair to their step in third periods.

For two periods, Thursday looked like a game where the Islanders very much missed Barzal as the Penguins controlled the neutral zone, forechecked hard and made it impossible for the Isles to get pucks deep.

It looked like a straightforward, and maybe even dominant, Penguins win.

But then again, so did the last two matches between these teams at the same juncture.