The incident happened at around 6.12pm last night (July 7) along Westow Street in Crystal Palace.
During the search, officers allegedly found a firearm, two knives and a quantity of suspected Class B drugs.
A Metropolitan Police spokesperson said: "Officers on patrol on Westow Street in Crystal Palace on Tuesday, 7 July carried out a search of three men at around 18:12hrs.
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"Police located a firearm, two knives and a quantity of suspected class B drugs.
"During the search, the men became obstructive by trying to flee and pull away from the scene before trying to encourage a dog to attack officers.
"The men, two aged 25 and a 19-year-old man, were arrested on suspicion of possession of a firearm, possession of a class B, assault and obstructing a police officer.
"They remain in custody.