The raiders, who worked together to plan and carry out the heist in Ashington, have been jailed for a combined total of more than 24 years.
Philip Wright, 51, Adam Rae, 28, and Tony Eastlake, 34, have been handed prison sentences for their part in the robbery at the Humber Terrace post office on August 15 last year.
Staff were threatened and forced to hand over almost £3,000.
A court was told how Eastlake carried out initial reconnaissance for the raid.
He then contacted Wright and Rae, who carried out the robbery by arriving at the scene on a moped and entering the shop with masks and gloves, threatening the two female staff members.
Detectives carried out an investigation that included CCTV enquiries, and Wright’s DNA was found on a wig he had worn to conceal his identify.
Wright, of Beatrice Street, Ashington, admitted robbery and possessing an imitation firearm with intent to cause fear of violence. He was sentenced to 11 years and three months.
Rae, of Moorland Court, Bedlington, admitted robbery and possessing an offensive weapon – a bottle containing an unknown substance – and was jailed for seven years and two months.
Eastlake, 34, of Katherine Street, Ashington, admitted conspiracy to rob and was jailed for six years.
Following the sentencing at Newcastle Crown Court, Northumbria Police's Detective Constable Colin Davies said he was pleased with the sentences.
"This was a professionally planned robbery with the offenders waiting until the store had received a cash drop-off before making their move,” he said.
“I want to thank the members of the community for their help with this investigation. Thankfully crimes such as these are extremely rare in our force area, but when they do occur we are committed to doing everything we can to bring offenders to justice.
"The offenders are now spending a significant period of time behind bars and hopefully that comes as some comfort to their victims who have been badly affected by the incident.”