John Gill Commentary Judges 3:1

"Now these are the nations which Jehovah left, to prove Israel by them, even as many [of Israel] as had not known all the wars of Canaan;" — Judges 3:1 (ASV)

Now these [are] the nations which the Lord left to prove
Israel by them
Which are later mentioned, (Judges 3:3Judges 3:5) ;

[even] as many [of Israel] as had not known all the wars of Canaan ;
those that Joshua, and the people of Israel under him, had with the Canaanites, when they first entered the land and subdued it; being then not born, or so young as not to have knowledge of them, at least not able to bear arms at that time.