Police have launched an investigation into the attack which took place in Sandall Park earlier this week.
A spokesman for South Yorkshire Police said: “Police were called shortly before 7pm on Monday 14 March following reports that three teenage boys had been robbed in Sandall Park.
“It is reported the victims were in the park, when they were approached by three men wearing balaclavas, who threatened them with a knife and made demands for their property.
"The men reportedly punched the victims, before making off with a mobile phone and a wallet.
“Enquiries are continuing and anyone with information which may assist police is asked to call 101 quoting incident number 838 of 14 March.
You can also call Crimestoppers to give information anonymously on a dedicated reporting hot line on 0800 555 111.