prova cosi:
html, body{ margin:0; padding:0; background-color:#000000; }
#contenitore{ width:100%; background-color:#DBDBDB; }
#banner { width:100%; width:155px; }
#col_1{ width:60%; height:400px; float:left; background-color:#FFFFFF; }
#col_2{ width:15%; height:400px; float:left; background-color:#FFFFFF; }
#col_3{ width:25%; height:400px; float:left; background-color:#FFFFFF; }