Bede

Saul, however, taking three thousand chosen men, etc. Just as in a good sense the number three is usually placed for a certain mysterious grace, clearly designating those who are either glorious by the confession of the Holy Trinity, or devoted by faith, hope, and charity, or perfect in thought, speech, and action, or something of the sort; so also, when it is placed in an evil army, it not undeservedly denotes those who are found to do everything contrary. …

Commentary on Samuel ↗