A man caught with two pistols and ammunition claimed he was in Glasgow for a painting and decorating job.
Royal Mail uniforms and council hi-vis vests were also seized during the raid at an Airbnb property in the east end.
Earlier armed officers had arrested two of the man's associates outside The Forge shopping centre.
Richard Carty, Ezekial Aremu and Cade Johnson admitted their part in a conspiracy to become involved in a serious organised crime.
The court heard that a large police operation began with officers monitoring the movements of the three men.
They observed the group had stayed at the Airbnb in Springfield Gardens from 7 March into the following day.
Just after midday, Aremu and Johnston, both 21 and from Prestwich, Bury, and another man were stopped as they drove a grey Mercedes car along East Wellington Street near the Forge shopping centre.
After they were removed from the car at gunpoint officers found a balaclava in the passenger footwell, £1,000 of counterfeit £20 notes in the glove boxes and two cans full of petrol in the boot.
Later that afternoon armed police went to the flat in Springfield Gardens where they found Carty, 38, from Salford, Manchester.
He was asked if there were any firearms and replied: "There is maybe something in a bag in there."
In the bedroom police found a holdall which contained a Luger pistol and six live cartridges and parts of a Glock pistol.
Royal Mail uniforms and bags and Glasgow City Council hi-vis vests were also recovered
Carty's DNA and Aremu's fingerprints and were found on the holdall and on a box that had been specially adapted to hold a gun.
Prosecutor Murdo McTaggart said: "The evidence in this case shows that Carty and Aremu were involved in the planning and preparation for a serious crime involving the use of a firearm with live ammunition.
"Aremu's position is that he was not going to be involved in the ultimate use of the weapons.
"Johnson was involved in the planning and preparation for a serious crime, although it is accepted he was not aware of the presence of the firearms."
The prosecutor said the target or targets of the planned attack had not been identified by the police,
He added: "The serious crime intended must have involved the use of disguises - a postman and a Glasgow City employee - with the intention of evading security of avoiding suspicion, to enable a place or person to be approached without suspicion."
Judge Lord Arthurson deferred sentence on all three until next month for background reports and remanded them in custody.
Defence counsel will give their pleas in mitigation then.