A basic three column layout will have the following structure:
<body>
<div id="outerWrapper">
<div id="contentWrapper">
<div id="leftColumn1"><left column content></div>
<div id="rightColumn1"><right column content></div>
<div id="content"><main content></div>
</div>
</div>
</body>
You can reorder the divs in the code to have the content div listed first:
<body>
<div id="outerWrapper">
<div id="contentWrapper">
<div id="content"><main content></div>
<div id="leftColumn1"><left column content></div>
<div id="rightColumn1"><right column content></div>
</div>
</div>
</body>
This change will not effect the layout.