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
psa 3:1For the historical occasion mentioned, compare 2Sa. 15:1-17:29. David, in the midst of great distress, with filial confidence, implores God's aid, and, anticipating relief, offers praise. (Psa 3:1-8)
Lord . . . increased--The extent of the rebellion (Sa2 15:13) surprises and grieves him.
psa 3:2say of my soul--that is, "of me" (compare Psa 25:3). This use of "soul" is common; perhaps it arose from regarding the soul as man's chief part.
no help . . . in God--rejected by Him. This is the bitterest reproach for a pious man, and denotes a spirit of malignant triumph.
Selah--This word is of very obscure meaning. It probably denotes rest or pause, both as to the music and singing, intimating something emphatic in the sentiment (compare Psa 9:16).
psa 3:3But--literally, "and" (Psa 2:6). He repels the reproach by avowing his continued trust.
shield--a favorite and often-used figure for protection.
my glory--its source.
lifter up of mine head--one who raises me from despondency.
psa 3:4cried . . . heard--Such has been my experience. The latter verb denotes a gracious hearing or answering.
out of--or, "from."
his holy hill--Zion (Psa 2:6). His visible earthly residence.
psa 3:5the Lord sustained me--literally, "will sustain me," as if his language or thought when he laid down, and the reason of his composure.
psa 3:6ten thousands of people--or, "myriads," any very great number (compare Sa2 16:18).
psa 3:7Arise, O Lord--God is figuratively represented as asleep to denote His apparent indifference (Psa 7:6). The use of "cheekbone" and "teeth" represents his enemies as fierce, like wild beasts ready to devour (Psa 27:2), and smiting their cheekbone (Kg1 22:24) denotes violence and insult.
thou hast broken--God took his part, utterly depriving the enemy of power to injure.
psa 3:8An ascription of praise to a delivering God, whose favor is an efficient benefit.