Io suddividerei i tre colonne, così:
codice:
#container{background-color:cyan;}
#container div{float:left; height:auto;}
#left,#right{width:25%}
#middle{width:50%}
#container div div{width:100%; height:100px; margin-bottom:10px;}
#uno{background-color:maroon}
#due{height:200px !important; background-color:green;}
#tre{background-color:red;}
#quattro{background-color:yellow;}
#cinque{background-color:navy;}
.push{clear:both;}
codice:
<div id="container">
<div id="left">
<div id="uno">1</div>
<div id="tre">3</div>
</div>
<div id="middle">
<div id="due">2</div>
</div>
<div id="right">
<div id="quattro">4</div>
<div id="cinque">5</div>
</div>
<br class="push" />
</div>
ciao, bio.