Witnesses described young clubbers racing for the exits at Lanterna Azzurra, in the town of Corinaldo, after an unknown substance was released into the crowd triggering a stampede.
The suspected use of a pepper spray-like substance is thought to have sparked the chaos at the venue in the town of Corinaldo, near Ancona in central Italy.
Carabinieri paramilitary police Col. Cristian Carrozza said the dead teenagers' ages ranged from 14 to 16 and the woman was 39.
Five children are among six people who have died in a stampede at an Italian nightclub, police have said.
Authorities did not immediately say how old the victims were, but RAI state radio said the mother was 40, and that about 1,000 people were inside when the stampede began.
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Shkodran Mustafi and Sead Kolasinac are also seen in the video with balloons in their mouths, which is how the gas is ingested. It is not illegal to possess nitrous oxide but it is illegal to supply it under the 2016 Psychoactive Substances Act.
Around 1,000 revellers are believed to have been in the club. People fell, one after the other, on top of each other.
They said 12 of the injured were in serious condition.
ANSA earlier had reported that someone had sprayed an irritant, which triggered the panic.
"People can not die like this", said Deputy Prime Minister Matteo Salvini, who also serves as interior minister. It quoted one teenager as saying that the emergency exit was blocked. It wasn't immediately clear how many people were inside when the stampede erupted or the club's maximum capacity. He said that when rescuers arrived, all the doors were open. Another eyewitness said a wall inside the club had collapsed in the melee.
"You can't die like this at 15", said far-right Interior Minister Matteo Salvini, promising to "find those responsible for these six broken lives, those who through wickedness, stupidity or greed turned a festive evening into a tragedy". That could have made it more likely that young teenagers were at the disco in the wee hours.