menu levels

hi people,

just a quick question. i have this code from internet to display a menu but i want to add a 3rd level menu... how do i make it please???

Code:

---

#navigation {
        margin:0;
        padding: 0;
        clear:both;
        width:500px;
        height:51px;
        background: #d6eaf8 url(images/dropdown-bg.gif) repeat-x left top;
}


ul.nav-main,
ul.nav-main li {
        list-style: none;
        margin: 0;
        padding: 0;
}


ul.nav-main {
        position: relative;
        z-index: 597;
}

ul.nav-main li:hover > ul {
        visibility: visible;
}


ul.nav-main li.hover,
ul.nav-main li:hover {
        position: relative;
        z-index: 599;
        cursor: pointer;
        background: url(images/dropdown-bg-hover.gif) repeat-x left top;
}



ul.nav-main li {
        float:left;
        display:block;
        height: 51px;
        color: #999;
        font: 14px Arial, Helvetica, sans-serif;
        background: url(images/separator.gif) no-repeat right center;
}

ul.nav-main li a {
        display:block;
        padding: 16px 16px 0 16px;
        height: 35px;
        color: #FFF;
        font: 14px Arial, Helvetica, sans-serif;
        text-decoration:none;
}

ul.nav-main li a:hover {
        color:#FFFFFF;
}



ul.nav-main *.list {
        background: url(images/navigation-arrow.gif) no-repeat right top;
}



ul.nav-sub {
        visibility: hidden;
        position: absolute;
        padding:10px;
        top: 48px;
        left: 0;
        z-index: 598;
        background: #353535;
        border-right: 1px solid #000;
        border-bottom: 1px solid #000;
        border-left: 1px solid #000;
}


ul.nav-sub li {
        list-style:none;
        display:block;
        padding: 0;
        height: 27px;
        float: none;
        width:145px;
        border-bottom: 1px solid #5a5a5a;
        background: none;
}

ul.nav-sub li a {
        list-style:none;
        display:block;
        color: #FFF;
        padding: 6px 5px 6px 5px;
        height: 15px;
        float: none;
        width:145px;
        background: none;
        font: 12px Arial, Helvetica, sans-serif;       
}

---

Thanks:
Duarte V.