Officers attended the incident at the Subway at Tulketh Mill on Blackpool Road just after 9.45pm on Monday August 21.
The men, who were dressed in black, were reported to have entered the shop with their faces covered carrying a knife and a claw hammer. They then demanded cash from the till and threatened the lone member of staff, say detectives.
The robbers then made off taking the till which contained around Â£100, they added.
A police spokesman said: "We were called to reports of a robbery at the Subway on Blackpool Road.
"Three men are reported to have entered the store armed with a knife and a hammer, before threatening a member of staff.
"They made off with a small quantity of cash.
"This incident has obviously left the worker very shaken and I would appeal to anyone in the area at the time who might have seen something to make contact with police.
A spokesman for Subway said: "We can confirm that this incident took place and the staff member working in the Subway store was thankfully unharmed.
"We are doing everything that we can to assist the police with their enquiries.''
Anyone with information about the robbery can contact police on 101 quoting log 1431 of August 21.