WITH THE THIRD INFANTRY DIVISION, in Central Iraq, March 26 — United States Army troops encircled the city of Najaf today after seizing three bridges across the Euphrates River. The clashes were the fiercest for the Third Infantry Division since the war began. According to American estimates, about 1,000 Iraqis have been killed in fighting near Najaf in the last three days. "What I've done is surround the city and cut it off," the division's commander, Maj. Gen. Buford C. Blount III, said in an interview today. The effort to encircle Najaf began at midnight Monday and lasted more than...