John Gill Commentary Judges 3:4

"And they were [left], to prove Israel by them, to know whether they would hearken unto the commandments of Jehovah, which he commanded their fathers by Moses." — Judges 3:4 (ASV)

And they were to prove Israel by them
They were left in the land, as to inure them to war, and try their courage, so to prove their faithfulness to God:

to know whether they would hearken to the commandments, of the Lord ,
which he commanded their fathers by the hand of Moses ;
even all the commandments of the Lord delivered to them by Moses, moral, civil, and ceremonial, and particularly those that concerned the destruction of the Canaanites, their altars, and their idols, (Deuteronomy 7:1Deuteronomy 7:5) .