G.I.'s Pry Iraqis Loose And Surge Over River
Army's Third Infantry Division clears last geographic obstacle to Baghdad, 20 miles away, after it seizes damaged bridge over Euphrates River following intense three-hour battle against more than company of dug-in Iraqi soldiers; thousands of American soldiers surge across bridge in tanks, Bradleys, mobile artillery batteries and array of other armored vehicles; there are no immediate reports of American casualties; battle for bridge described; Iraqi tactics that proved efective against forces encircling Najaf, Nasiriya and Samawa--sniping, ambushes, taxi bomb--could do little to slow division's armored forces; photo (M)