Police believe that the three offenders may have been responsible for a similar attack shortly before the robbery.
Detectives have received a report of an assault in the same location, which is believed to involve the same group of men.
A 17-year-old boy was punched in the face but managed to run off.
Officers are conducting enquiries in the area and would like to speak to anyone who may know who was responsible.
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Dry with clear spells overnight. Dry and breezy with sunny spells for much of tomorrow. Rain arriving later. Becoming humid from Monday.
Showers fading away with clear spells overnight. Dry with sunny spells for much of tomorrow with rain arriving later. Becoming humid later.
Feeling warm in this afternoon's sunshine. Breezy at times with the risk of an isolated shower.