/* make the second level visible when hover on first level list OR link */
.header_menu ul li:hover ul,
.header_menu ul a:hover ul{display:none;}
/* keep the third level hidden when you hover on first level list OR link */
.header_menu ul :hover ul ul{display:none;}
/* make the third level visible when you hover over second level list OR link */
.header_menu ul :hover ul :hover ul{display:none;}