Outside Madison Square Garden, it was sunny and beautiful, a picture-perfect day for the biggest Knicks home playoff game in over two decades. 

Those conditions didn’t extend into the arena.

Inside, it was gloomy, a powerful storm of Celtics 3-pointers that wouldn’t stop until this Eastern Conference semifinal series had a vastly different feel to it. 

The Knicks may still lead this series 2-1, but that advantage feels incredibly tenuous after the Celtics’ 115-93 Game 3 pounding inside a quiet and dejected MSG. 

This looked like the first three regular-season games between the two teams — three Celtic romps.

This more closely resembled the Celtics team that was supposed to cruise past the Knicks into the Eastern Conference finals for the fourth straight year and was considered a favorite to repeat as champions. 

Entering Saturday, the Celtics were the 31st team in NBA playoff history to drop the first two home games in a best-of-seven series.

Only five of those have gone on to win the series. Boston brought the intensity of a group that is intent on becoming the sixth team — and the Knicks didn’t come close to matching that effort. 

“I don’t think we came with the mindset of being satisfied, but I think [we were] subconsciously satisfied with being up 2-0,” Jalen Brunson said. “But it’s not [the way] we needed to approach the game.”

After getting shut down in Boston, All-Star wings Jayson Tatum and Jaylen Brown found their game, combining for 41 points, 15 rebounds and 12 assists.

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The Celtics hit 20 3-pointers on 40 attempts after missing a combined 75 over the first two contests.

It was a 25-point game at halftime and 31 early in the third quarter. There was extended garbage time, as Marv Albert liked to say. 

Several Knicks said their intensity was lacking, that they didn’t bring the needed urgency into the third game of the series.

Coach Tom Thibodeau and Josh Hart pointed to a sluggish offensive start negatively impacting them at the other end of the floor. Their pick-and-roll defense was shaky, leading to far too many open looks for the Celtics. 

“When it comes to our pick-and-roll defense, we are going to have our covers and we’re going to stick to what we do and we’re going to adjust when we need to adjust, but everything we need to do has to be with more intensity and more urgency and more force,” Brunson said. “Just from that standpoint, it’s not really schematically, it’s more, ‘Do we want it?’ And tonight obviously that wasn’t the answer.”

Unlike the first two games of the series, the Knicks weren’t coming back from that kind of deficit again. They never got closer than 20, setting up a pivotal Game 4 back at the Garden Monday night. 

Midway through the third quarter, the Knicks were slowly starting to build momentum. The lead was 24 when Brunson inexplicably dribbled the ball off his foot. It led to a Tatum layup in transition — symbolic of the lost afternoon. 

“They came out, set the tone and put us on our heels,” Mikal Bridges said. 

Really, nobody played well for Thibodeau’s team.

OG Anunoby (two points) didn’t score in the first half and has seven points over the past two games. Brunson struggled over the first three quarters.

The rebounding was poor early and the defense was soft. Mitchell Robinson’s issues at the free-throw line continued with a 4-of-12 showing, dropping him to 11-of-39 in the playoffs.  

Brunson led the Knicks with 27 points, but most of that came with the game well in hand.

Karl-Anthony Towns added an impact-free 21 points and 15 rebounds and appeared to injure a finger on his left hand in the third quarter. He declined to say if he needed an X-ray afterward and did finish the game. Payton Pritchard scored 23 points off the bench for Boston. 

The Celtics made their first four 3-point tries, and kept on drilling them from deep. They were 12-of-19 from distance in the first half and led by 25 at halftime. 

The large crowd sounded like a library much of the afternoon, matching the underwhelming Knicks effort. With a chance to bury the Celtics, they didn’t show up. 

“I think we might have let human nature get to us a little bit,” Bridges said.