The robbery happened at the Belmont Road premises in east Belfast after 8 pm on Sunday evening.
Police say the men threatened staff and made off with the cash and handbags belonging to two members of staff.
The gang are believed to have escaped along Dundela Avenue in a red-coloured Mazda driven by a third man.
The car thought to have been used in the getaway was later discovered at Stratheden Street.
No-one was injured in the incident, but the staff members were badly shaken.
Police have issued descriptions of the suspects.
One of the men is described as being approximately 5ft 9in tall and of stocky build.
The second man was also of stocky build, around 5ft 6in to 5ft 7in tall, and was wearing a powder-blue jacket and blue jeans.
Anyone with information about the robbery is asked to contact detectives at Strandtown CID on 0845 600 8000.