Ice Cube insisted there was a supply of relatively spry former NBA players looking for some action, and a demand to see them from their old fans.

On Sunday, the rapper/actor’s well-honed business sense proved sharp again. His “BIG3” halfcourt, three-on-three league debuted at Barclays Center before an enthusiastic crowd of about 15,177.

That is not far behind the Nets’ average of 15,429 last season, and the vibe was far livelier than that at your average Nets game, sustained for most of a show that lasted nearly six hours and featured four games.

If there was a downer, it was that fans did not see enough of the main attraction, 3’s Company player / coach Allen Iverson.

The Hall of Famer had warned on Friday not to expect too much of him at age 42, seven years removed from his last NBA game. He wasn’t kidding.

Iverson shot 1-for-6 for two points and added two assists in only nine minutes of action, but he now is 1-0 as a coach, because 3’s Company beat the Ball Hogs, 61-51.

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“The playing part is not going to be what you expect,” he said afterward. “I’m 42 years old. I’ve been retired for six, seven years. The only reason I get out there for the couple of minutes that I do get out there is for the fans . . . You’re not going to see the Allen Iverson of old out there.”

Iverson knew fans would clamor to see him play, so he started himself. When they later started chanting, “We want A.I.,” he put himself back in, but only briefly.

“Man, it was just fun being out there in the atmosphere,” he said. “You get flashbacks of when you were that guy and everybody was coming to see you perform, not just show up.”

Mostly Iverson and everyone else involved seemed pleased to be a part of something that got off to a bigger start than almost anyone anticipated.

“This is going to be something special,” said Clyde Drexler, coach of a team called Power. “Outside the NBA, this is the most competitive basketball league in the world.”

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Perhaps in halfcourt basketball it is. But part of the point is to allow men in their late 30s and early 40s to do their thing without having to run up and down a full court.

The league will feature eight teams playing quadrupleheaders every Sunday this summer. Next are stops in Charlotte, Tulsa and Philadelphia.

The games are untimed, with the winner being the first to 60 and ahead by at least two. Among the rules are three “four-point shot” circles beyond the arc. Former Knick Mike Bibby of Ghost Ballers made two of them.

BIG3 allows hand-checking, unlike the NBA, and the style of play was extremely rugged throughout the first afternoon.

Rashard Lewis’ free throw to complete a three-point play gave 3 Headed Monster a 62-60 victory over Ghost Ballers in the first game. Lewis punctuated his basket with a barrage of trash talk aimed at Ricky Davis.

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Deshawn Stevenson’s three-pointer won the second game for Power, 62-60, over coach Julius Erving’s Tri-State.

The most anticipated game, between 3’s Company and the Ball Hogs, was unsightly. Fans started to boo loudly after a long, uninterrupted stretch of bricked shots.

In the final game, coach Charles Oakley’s Killer 3s got killed by Trilogy, 60-45.

Most players seemed to be in decent physical shape, but Jason (White Chocolate) Williams, Corey Maggette, Kenyon Martin and Rashad McCants left their games with injuries.

Assorted celebrities were in the house, including LL Cool J and Whoopi Goldberg. But the audience that Ice Cube was most focused on is current and recently retired NBA stars.

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Among those at Barclays were young pros such as James Harden and new Net D’Angelo Russell and recently retired Paul Pierce.

To sustain and grow interest, BIG3 will need infusions of big-name talent in coming years. Pierce is high on Ice Cube’s priority list for 2018.

Other players said Pierce told them how impressed he was. But many sports leagues have strong starts, including the XFL. Sustaining it is the trick.

So far, so good. Said Iverson: “The sky’s the limit.”