Three boys have allegedly held a boy, 16, in a headlock and punched him on the head in an attempted robbery on Sydney's north shore, police say.

The victim was on Archer Street, Chatswood, walking home from school, when one of the alleged attackers approached him and held him in a headlock about 2.30pm yesterday, police said.

The other boys then allegedly punched him on the head, telling him they would not stop until he gave them money.

When he told the boys he had no money, they allegedly forced him to walk to an ATM to withdraw cash.

But a witness to the alleged attack phoned the police, and officers arrested the three boys - aged 14, 15 and 16 - as they walked to the ATM.

Officers also allegedly found cannabis on the boys.

The alleged victim was treated by paramedics and taken to Royal North Shore Hospital with suspected concussion.

The boys were charged with assault, with attempting to rob in company, possessing a prohibited drug and stealing.

They were given conditional bail and will appear in Hornsby Children's Court on March 8.