John Gill Commentary Judges 3

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 theland and subdued it; being then not born, or so young as not to have knowledge of them, at least not able tobear arms at that time.

Only that the generations of the children of Israel might know
and teach them war

That is, the following nations were left in the land, that the young generations of Israel might by their wars and conflicts with them learn the art of war, and be inured to martial discipline; which, if none had been left to engage with, they had been ignorant of: besides, their fathers in Joshua's time, as Jarchi and Kimchi observe, had no need to learn the art of war, for God fought for them; they did not get possession of the land by their own arm, and by their sword, but by the power of God in a miraculous way; but now this was not to be expected, and the Canaanites were left among them to expel, that they might be trained up in the knowledge of warlike affairs, and so be also capable of teaching their children the military art; which they should make use of in obeying the command of God, by driving out the remains of the Canaanites, and not give themselves up to sloth and indolence;

though some think that the meaning is, that God left these nations among them, that they might know what war was, and the sad effects of it; and the difference of fighting with their enemies alone, as other men, and the Lord fighting along with them, and for them, as he did for their fathers:

at least such as before knew nothing thereof ;
being either unborn, or at an age incapable of bearing arms, or learning the art of war.

[Namely], five lords of the Philistines
The places they were lords of were Gaza, Ashdod, Ashkelon, Gath, and Ekron; see (Joshua 13:3); three of these, Gaza, Ashkelon, and Ekron, had been taken from them by Judah, since the death of Joshua, (Judges 1:18); but they soon recovered them again, perhaps by the help of the other two. The Philistines were a people originally of Egypt, but came from there and settled in these parts, and were here as early as in the times of Abraham, and were very troublesome neighbours to the Israelites in later times; see (Genesis 10:14) (Genesis 21:32Genesis 21:33);

and all the Canaanites ;
these were a particular tribe or nation in the land so called, which inhabited by the sea, and by the coast of Jordan, (Numbers 13:29); otherwise this is the general name for the seven nations:

and the Sidonians ;
the inhabitants of the famous city of Sidon, which had its name from the firstborn of Canaan, (Genesis 10:15);

and the Hivites that dwelt in Mount Lebanon ;
on the north of the land of Canaan:

from Mount Baalhermon ;
the eastern part of Lebanon, the same with Baalgad, where Baal was worshipped:

unto the entering in of Hamath ;
the boundary of the northern part of the land, which entrance led into the valley between Libanus and Antilibanus; see (Numbers 34:8) (Joshua 11:3) (13:5).

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) .

And the children of Israel dwelt among the Canaanites
As if they had been only sojourners with them, and not conquerors of them; and dwelt by sufferance, and not as proprietors and owners; such were their sloth and indolence, and such the advantage the inhabitants of the land got over them through it, and through their compliances with them; and this was the case not only of one sort of them, the Canaanites, but of the rest:

the Hittites, and Amorites, and Perizzites, and Hivites, and
Jebusites ;
who all had cities in the several parts of the land, with whom the children of Israel were mixed, and with whom they were permitted to dwell.