A loaded gun and a "significant" amount of bullets were recovered after police raided containers in Miles Platting.
Police have been at the scene between Viaduct Street and Barking Street for three days with armed officers guarding the location overnight.
An operation was launched early on Wednesday morning in an industrial area of the district. A yard full of metal cabins was targeted.
In a statement Greater Manchester Police said: "On Wednesday February 17th, officers from GMP's City of Manchester Intelligence Development Team executed a warrant at an address in Barking Street, Manchester.
"Three men, aged 30, 41, and 52, have been questioned on suspicion of firearms offences and money laundering offences. They remain in custody.
"During the warrant a loaded firearm was recovered, along with a significant amount of ammunition.
"A significant amount of cash was also recovered during the investigation. Enquiries remain ongoing."
One local said: 'At 8am on Wednesday they did a police raid in the cabins. They left about 4pm and then came back to guard it at night.
"Armed police have been on my street since Wednesday. They have been sat there all night guarding the cabin grounds."
Another said: "There was armed police surrounding this place. They were waiting for search teams to come down so they can assist them search all the containers in the yard in part of an ongoing investigation. "
It is understood that tens of thousands of pounds in cash was seized in the raid and a BMW three series, 19 plate, car was also recovered.
The raid and seizures are not being linked at this stage to recent shootings in Harpurhey and the Abbey Hey area of Gorton but police are "keeping an open mind".
