Adam Clarke on Judges 3:1

Now these are the nations - The nations left to prove the Israelites were the five lordships or satrapies of the Philistines, viz., Gath, Askelon, Ashdod, Ekron, and Gaza; the Sidonians, the Hivites of Lebanon, Baal-hermon, etc.; with the remains of the Canaanites, viz., the Hittites, Amorites, Perizzites, and Jebusites. Those who were left to be proved were those Israelites that had not seen all the wars of Canaan.