After the last add div overflow: hidden; follows:
<div style="width:580px; height:auto; margin:0 auto;
<div style="height:100px; width:230px; float:left; margin:20px"></div>
<div style="height:100px; width:230px; float:left; margin:20px"></div>
<div style="height:100px; width:230px; float:left; margin:20px"></div>
<div style="clear:both;height:1px;width:100%; overflow:hidden; margin-top:-1px;"></div>
</div>
Personally verify.