Charles Ellicott 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)

To prove Israel. —The verb here used is the same as in Judges 2:22 and Judges 3:4, but, as R. Tanchum observes, it is used in a slightly different sense, meaning “to train them.” Symmachus renders it askēsai.

As many of Israel as had not known all the wars of Canaan. —This expression clearly implies the generation after that of Joshua. “The wars of Canaan” are equivalent to “the wars of the Lord,” and refer to the struggles of the actual conquest.