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 and sunny with long spells of sunshine in the south east and feeling hot
Fine and dry with some good long sunny spells, especially this afternoon and early evening and feeling very warm
Saturday morning's weather for the east of the region