div#menu {width: 160px;line-height:22px;text-indent:10px;margin: 0 auto; 

}
div#menu a{color:#FFFFFF;}
div#menu ul {padding: 0; width: 160px; margin:0px;}
div#menu li.sousmenu {background:#3b1abd;}
div#menu ul li {position:relative; list-style: none; }
div#menu ul ul {position: absolute; top: 0; left: 160px; display:none;background:#3b1abd;}
div#menu li a {text-decoration: none;}
div#menu li.sousmenu a:hover {text-decoration: none;background:#FF0000;}

div#menu ul.niveau1 li.sousmenu:hover ul.niveau2, 
div#menu ul.niveau2 li.sousmenu:hover ul.niveau3 {display:block; width: 250px;}