“3,000 chariots.” The Hebrew text reads 30,000, but it is well recognized that this is a copyists error. A version of the Septuagint, and the Syriac version, reads 3,000, which is much more likely correct. Or, it could be that the Hebrew word “thousand,” which can also refer to a fighting unit (“unit,” “squad,” “platoon”) could mean that here. In that case, the Philistines could have had 30 units of chariots. If there were ten chariots in a unit, there would be 300 chariots. One reason any chariots at all is in question is that the terrain between Gibeah and Michmash is hilly, rocky, very uneven, and basically impossible terrain for chariot warfare. This is footsoldier terrain.
“and people as the sand that is on the seashore in multitude.” God promised Abraham that his descendants would be like the sand on the seashore in multitude (Gen. 22:17), but here the unbelieving Philistines are that numerous.
“east of Beth-aven.” The Hebrew is idiomatic: “in front of Beth-aven.”