Multiple decks on a three-story building collapsed during a firefighters’ event at the Jersey Shore — injuring 22, including at least three children.

The decks crashed down from a property in Wildwood at 6 p.m. Saturday during an annual New Jersey Firemen’s Convention, according to authorities.

Shocking video captured at the scene shows at least a dozen firefighters in full turnout gear standing around a huge slab of fallen deck as they work to free anyone trapped underneath.

Most of those watching on also wear t-shirts and hats marking local fire units attending the event that attracts thousands of current and former firefighters to the resort town.

Local hospital officials say at least 22 people — including three children — were treated for injuries.

Early reports said at least two people were being treated in trauma units, according to CBS Philly, although no fatalities have been reported.

Authorities are still investigating the cause of the collapse.

“It was horrible,” witness Gail Ivins told the station, saying everyone was sitting on the decks and not “dancing or anything.”

“We heard the noise of it starting to pull away,” she said. “People started sliding off and yelling, falling.

“It was very slow motion, it was really scaring. It came off in pieces.”

She said there was a 2-year-old girl still on the deck in a corner that did not fall, with men there ready to catch her when it did fall.

Another woman on a third-floor section was not so lucky and “slid off right onto the sidewalk,” Ivins told the station.

Neighbor Diamond Heard said it was especially upsetting that it happened during the convention.

“The worst thing about it is, people that are heroes, who are here vacationing,” Heard told CBS.

“They’re just down here chilling, having a good old time. The parade was just earlier. Those were the ones who were hurt.”

With Post Wires