AL-HUD, Iraq — The mutilated bodies of Islamic State group fighters were still strewn on the ground of this northern Iraqi town on Wednesday. One was burned. Another’s face was flattened by abuse. Iraqi troops on the march toward Mosul moved into al-Hud a day earlier and declared it liberated. But they found residents had already risen up and killed many of the militants in the town themselves. With the offensive to recapture Mosul in its third day, Iraqi forces advancing from the south and east are fighting to retake the towns and villages the dot the plains and line the Tigris River leading to the city.