I think it cannot be done by css nakedly, but can be done using some trick by using some css class.
Set css class to every next 3 alphabet li.
<style type="text/css">
ul {
list-style: none;
}
li {
float: left;
width: 20px;
text-align: center;
margin-left: 2px;
}
.down {
clear: both;
}
</style>
<ul>
<li>A</li>
<li>B</li>
<li>C</li>
<li class="down">D</li>
<li>E</li>
<li>F</li>
<li class="down">G</li>
<li>H</li>
<li>I</li>
<li class="down">J</li>
<li>K</li>
<li>L</li>
<li class="down">M</li>
<li>N</li>
<li>O</li>
</ul>
