Robbery suspects were traced to an address after a shop worker turned sleuth.
A 53-year-old woman was threatened by a man brandishing a knife as she served behind the counter at the One Store in Fox Street, Dawdon.
The attacker then grabbed cash from the till and ran off.
A 27-year-old shop worker followed the raider and
tracked him from a distance as he headed to a nearby house.
He alerted police, who went into the home and arrested two men on suspicion of robbery and a woman on suspicion of conspiracy to rob.
The three, all aged 33, were questioned by detectives last night and released on bail pending further inquiries.
Police have praised the "courageous" efforts of the shop worker.
Detective Sergeant Paul Colling, of Seaham CID, said: "He did a fantastic job.
"He knew the lad had a knife, gave foot chase and helped us identify the suspects. What he has done is to be commended.
"He's stood for what's right, not put himself at risk and has been sensible."
Police recovered cash as part of their investigations into the incident, which happened at 5pm on Wednesday.
They are appealing for any witnesses to come forward as inquiries continue.
Durham Constabulary can be called on 0345 60 60 365 or Crimestoppers anonymously on 0800 555 111.
