CSS Questions

Yes you can have a div within a div, in fact to any level you wish.

As to the three column layout, there are plenty of examples on the web but if you want equal height columns (which makes things easy if you have different coloured backgrounds for each column) then you need to wrap two of the columns in another div.

Code:

<div id="main">
   <div id="wrapper">
       <div id="nav">
       </div>
       <div id="content">
       </div>
   </div>
   <div id="ads">
   </div>
</div>

Then your CSS would float the wrapper div to the left along with its other attributes, one of which should be a positioning attribute of either relative or absolute so that the inner divs can be positioned relative to its coordinates again using float. The "ads" div is floated right.

HTH