Here's what I've got so far: fiddle
2 problems with it though:
- I've hard-coded the width of each lito33%, which I'd prefer not to do so that I can easily add more items.
- I want to put some spacing between each item (a gap in the background color), but as soon as I add a margin, one item will be bumped down a line. How do I get around that?
#main-nav {
    list-style: none;
    margin: 0;
    padding: 0;
    width: 100%;
    overflow:  auto;
}
#main-nav li {
    float: left;
    width: 33%;
    height: 25px;
    text-align: center;
}
#main-nav li a {
    width: 100%;
    display: block;
    height: 100%;
    line-height: 25px;
    text-decoration: none;
    background-color: #e0e0f0;
    font-weight: bold;
    color: #021020;
}
#main-nav li a:hover {
    background-color: #498cd5;
    color: #ddeeee;
}<ul id="main-nav">
    <li><a href="#">one</a></li>
    <li><a href="#">two</a></li>
    <li><a href="#">three</a></li>
</ul> 
     
     
    