I think they were simply "teaching him a lesson," as it is referred to.
Had it not been those, Don Vito Corleone may never have been "Don," as their attack gave Vito the confidence against Fanucci.
Mick
...dot com bold typeface rhetoric. You go clickety click and get your head split. 'The hell you look like on a message board Discussing whether or not the Brother is hardcore?
In case it's still bothering you, here the full quote from the novel itself:
"The three young men [who attacked Fanucci] were not murderers, merely tough young boys determined to teach him a lesson and stop him from scavenging [around the neighbourhood]."
"Mr. Corleone is a man who insists on hearing bad news immediately..."