Three teenagers have been charged over a robbery that left a convenience store employee with a suspected fractured skull.

The male youths entered the Campbelltown premises about 12.45am (AEDT) on Saturday, armed with a knife and a hammer and threatened the male employee, police say.

One of the youths allegedly threw the hammer at the worker, hitting him in the head and wounding him in the left eye.

Police say the offenders then stole a quantity of cash, confectionery and personal items before fleeing the shop.

Officers were called to the scene and a short time later arrested three teenagers in nearby Koshigaya Park.

A 16-year-old Claymore male was charged with robbery while armed and another 16-year-old Claymore boy was charged with dealing with the proceeds of crime.

A 15-year-old Airds boy was charged with robbery using an offensive weapon as well as charges relating to another incident.

The trio are due to appear at Parramatta Children's Court on Sunday.

The injured man was taken to Campbelltown Hospital with an eye injury and a suspected fractured skull.