In the following article you will find a brief commentary on Psalm 3 that goes along with our Riverside Church Two Year Bible Reading Plan (Volume 1 & Volume 2). This plan will allow you to read the New Testament and Psalms once every year and the Old Testament once every two years.
From Matthew Henry:
As the foregoing psalm, in the type of David in preferment, showed us the royal dignity of the Redeemer, so this, by the example of David in distress, shows us the peace and holy security of the redeemed, how safe they really are, and think themselves to be, under the divine protection. David, being now driven out from his palace, from the royal city, from the holy city, by his rebellious son Absalom,
I. Complains to God of his enemies (v. 1, 2).
II. Confides in God, and encourages himself in him as his God, notwithstanding (v. 3).
III. Recollects the satisfaction he had in the gracious answers God gave to his prayers, and his experience of his goodness to him (v. 4, 5).
IV.Triumphs over his fears (v. 6) and over his enemies, whom he prays against, (v. 7).
V. Gives God the glory and takes to himself the comfort of the divine blessing and salvation which are sure to all the people of God (v. 8).
Those speak best of the truths of God who speak experimentally; so David here speaks of the power and goodness of God, and of the safety and tranquility of the godly.
 Henry, M. (1994). Matthew Henry’s commentary on the whole Bible: complete and unabridged in one volume (p. 747). Peabody: Hendrickson.