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.
More top news
Southend United are promoted to League One after a nail-biting play-off final against Wycombe, which they won 7-6 on penalties.
Kate Haskell has the weekend forecast for the east of the region
Kate Haskell has the weekend forecast for the west of the region.