A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
jdg 3:1NATIONS LEFT TO PROVE ISRAEL. (Jdg 3:1-4)
these are the nations which the Lord left, to prove Israel--This was the special design of these nations being left, and it evinces the direct influence of the theocracy under which the Israelites were placed. These nations were left for a double purpose: in the first instance, to be instrumental, by their inroads, in promoting the moral and spiritual discipline of the Israelites; and also to subserve the design of making them acquainted with war, in order that the young, more especially, who were total strangers to it, might learn the use of weapons and the art of wielding them.
jdg 3:5BY COMMUNION WITH THESE THE ISRAELITES COMMIT IDOLATRY. (Jdg 3:5-7)
the children of Israel dwelt among the Canaanites--The two peoples by degrees came to be on habits of intercourse. Reciprocal alliances were formed by marriage till the Israelites, relaxing the austerity of their principles, showed a growing conformity to the manners and worship of their idolatrous neighbors.
jdg 3:8OTHNIEL DELIVERS ISRAEL. (Jdg 3:8-11)
sold them--that is, "delivered them"
into the hand of Chushan-rishathaim--or, Chushan, "the wicked." This name had been probably given him from his cruel and impious character.
served Chushan-rishathaim eight years--by the payment of a stipulated tribute yearly, the raising of which must have caused a great amount of labor and privation.
jdg 3:9when the children of Israel cried unto the Lord--In their distress they had recourse to earnest prayer, accompanied by humble and penitent confession of their errors.
Othniel--(See on Jos 15:16; Jdg 1:13). His military experience qualified him for the work, while the gallant exploits he was known to have performed, gained him the full confidence of his countrymen in his ability as a leader.
jdg 3:10The Spirit of the Lord came upon him and he judged Israel, and went out to war--Impelled by a supernatural influence, he undertook the difficult task of government at this national crisis--addressing himself to promote a general reformation of manners, the abolition of idolatry, and the revival of pure religion. After these preliminary measures, he collected a body of choice warriors to expel the foreign oppressors.
the Lord delivered Chushan-rishathaim king of Mesopotamia into his hand; and his hand prevailed against Chushan-rishathaim--No details are given of this war, which, considering the resources of so potent a monarch, must have been a determined struggle. But the Israelitish arms were crowned through the blessing of God with victory, and Canaan regained its freedom and independence.
jdg 3:11Othniel . . . died--How powerful the influence of one good man is, in church or state, is best found in his loss [BISHOP HALL].
jdg 3:12EHUD SLAYS EGLON. (Jdg. 3:12-30)
the children of Israel did evil again in the sight of the Lord--The Israelites, deprived of the moral and political influences of Othniel, were not long in following their native bias to idolatry.
the Lord strengthened Eglon the king of Moab--The reigning monarch's ambition was to recover that extensive portion of his ancient territory possessed by the Israelites. In conjunction with his neighbors, the Ammonites and the Amalekites, sworn enemies of Israel, he first subjected the eastern tribes; then crossing the Jordan, he made a sudden incursion on western Canaan, and in virtue of his conquests, erected fortifications in the territory adjoining Jericho [JOSEPHUS], to secure the frontier, and fixed his residence there. This oppressor was permitted, in the providence of God, to triumph for eighteen years.
jdg 3:15Ehud the son of Gera--descended from Gera, one of Benjamin's sons (Gen 46:21).
left-handed--This peculiarity distinguished many in the Benjamite tribe (Jdg 20:16). But the original word is rendered in some versions "both-handed," a view countenanced by Ch1 12:2.
by him the children of Israel sent a present unto Eglon the king of Moab--the yearly tribute, which, according to Eastern fashion, would be borne with ostentatious ceremony and offered (Jdg 3:18) by several messengers.
jdg 3:16Ehud made him a dagger . . . and he did gird it . . . upon his right thigh--The sword was usually worn on the left side; so that Ehud's was the more likely to escape detection.
jdg 3:19quarries--rather, "graven images" (Deu 7:25; Jer 8:19; Jer 51:52); statues of Moabite idols, the sight of which kindled the patriotic zeal of Ehud to avenge this public insult to Israel on its author.
I have a secret errand unto thee, O king: who said, Keep silence--"Privacy"--a signal for all to withdraw.
jdg 3:20a summer parlour--Hebrew, "chamber of cooling"--one of those retired edifices which Oriental grandees usually have in their gardens, and in which they repose during the heat of the day.
jdg 3:21Ehud put forth his left hand--The whole circumstance of this daring act--the death of Eglon without a shriek, or noise--the locking of the doors--the carrying off the key--the calm, unhurried deportment of Ehud--show the strength of his confidence that he was doing God service.
jdg 3:27he blew a trumpet in the mountain of Ephraim--summoned to arms the people of that mountainous region, which, adjoining the territory of Benjamin, had probably suffered most from the grievous oppression of the Moabites.
jdg 3:28they went down after him, and took the fords--(See on Jos 2:7). With the view of preventing all escape to the Moabite coast, and by the slaughter of ten thousand men [Jdg 3:29], Ehud rescued his country from a state of ignominious vassalage.
jdg 3:31after him was Shamgar--No notice is given of the tribe or family of this judge; and from the Philistines being the enemy that roused him into public service, the suffering seems to have been local--confined to some of the western tribes.
slew . . . six hundred men with an oxgoad--This instrument is eight feet long and about six inches in circumference. It is armed at the lesser end with a sharp prong for driving the cattle, and on the other with a small iron paddle for removing the clay which encumbers the plough in working. Such an instrument, wielded by a strong arm, would do no mean execution. We may suppose, however, for the notice is very fragmentary, that Shamgar was only the leader of a band of peasants, who by means of such implements of labor as they could lay hold of at the moment, achieved the heroic exploit recorded.