The mob, each with their hoods pulled tight round their faces, walked up to the window of Ramsdens Pawnbrokers and Jewellers before shattering the glass and snatching a selection of items.
Witnesses say it is possible the men used a hammer to shatter the glass before making off down an alley between a dentist and amusement arcade at The Green, Southwick.
Detective Inspector Gary Hetherington, from Northumbria CID, said: "They fled with a quantity of jewellery. We are still trying to determine how much it was all worth.
"It appears the window display had just been laid out before the incident took place.
"After grabbing the jewellery, they made off on foot. We do not believe there was a car involved."
Police believe there would have been a number of witnesses to the theft, which happened between 3pm and 3.10pm yesterday.
The first man is described as wearing a grey hooded top, 6ft tall and of slim build. He was also wearing dark trousers.
His accomplice had a blue hoodie pulled round his face. He was also wearing dark trousers and is described as 6ft.
The third man also wore a dark coloured hoodie, dark coloured trousers and is described as 5ft 8in tall.
Detectives were today expected to be trawling through CCTV footage in a bid to identify the men involved.
Det Insp Hetherington added: "This incident happened in the middle of the afternoon so it's possible there were a number of people about at the time."
The theft follows a string of similar incidents at The Green since the beginning of the year.
In April, a bookie was held up by two men allegedly armed with baseball bats as he opened the back door of a betting shop. The masked men lay in wait at the back of William Hill bookmakers at The Green.
Just a week earlier, a gang wearing ski masks walked into the Coral betting shop, also at The Green, allegedly armed with baseball bats and stole cash.
Anyone with information about the latest theft is asked to contact Sunderland CID, tel. 454 7555 ext 66445.