These are the nations the LORD left to test all those Israelites who had not experienced any of the wars in Canaan
References for Judges 3:1
(he did this only to teach warfare to the descendants of the Israelites who had not had previous battle experience):
the five rulers of the Philistines, all the Canaanites, the Sidonians, and the Hivites living in the Lebanon mountains from Mount Baal Hermon to Lebo Hamath.
References for Judges 3:3
They were left to test the Israelites to see whether they would obey the LORD’s commands, which he had given their ancestors through Moses.
References for Judges 3:4