And three of the thirty - The word שלשים shalishim, which we translate thirty, probably signifies an office or particular description of men. Of these shalishim we have here thirty-seven, and it can scarcely be said with propriety that we have thirty-seven out of thirty; and besides, in the parallel place, 1 Chronicles 11:11-47, there are sixteen added. The captains over Pharaoh's chariots are termed שלשים shalishim, Exo 14:7.
The Philistines pitched in the valley of Rephaim - This is the same war which is spoken of Sa2 5:17, etc.

And David was then in an hold,.... In a strong hold; the strong hold of Zion, as Josephus (x), or one on a rock near the cave of Adullam, see Ch1 11:15,

and the garrison of the Philistines was then in Bethlehem; which was about six miles from Jerusalem; the valley of Rephaim lay between that and Bethlehem; so far had they got into the land of Judea, and such footing in it, as to have a garrison so near its metropolis.

(x) Antiqu. l. 7. c. 12. sect. 4.