Chronologically these verses describe what will follow verse 3.
14:12 The Lord would smite the nations that warred against Jerusalem (vv. 1-3) with a plague that would cause the people's flesh to rot off them wherever they might be. They would not be able to see or speak (cf. Isa. 37:36).
14:13 Panic would seize them from the Lord in that day, and they would fight one another (cf. Judg. 7:22; 1 Sam. 14:15-20; 2 Chron. 20:23). This would happen near Jerusalem.
14:14 The Israelites would also fight their enemies there and would gather much spoil from the people they would defeat. Thus there are three instruments God would use to defeat Israel's enemies: plague (v. 12), themselves (v. 13), and the Israelites (v. 14).
14:15 The plague that the Lord would send on Israel's enemies (v. 12) would also afflict their animals precluding their escape (cf. Josh. 7:24-25).
