You can get a liquid three-column loading by simply laying out three divs that are equal to the highest possible grid that is .span12. So in this case we used three divs, 2x sidebar .span3divs + 1x .span6content div and still 12.
HTML
<div class="navbar navbar-fixed-top">
<div class="navbar-inner">
<div class="container-fluid">
<a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</a>
<a class="brand" href="#">Project name</a>
<div class="btn-group pull-right">
<a class="btn dropdown-toggle" data-toggle="dropdown" href="#">
<i class="icon-user"></i> Username
<span class="caret"></span>
</a>
<ul class="dropdown-menu">
<li><a href="#">Profile</a></li>
<li class="divider"></li>
<li><a href="#">Sign Out</a></li>
</ul>
</div>
<div class="nav-collapse">
<ul class="nav">
<li class="active"><a href="#">Home</a></li>
<li><a href="#about">About</a></li>
<li><a href="#contact">Contact</a></li>
</ul>
</div>
</div>
</div>
</div>
<div class="container-fluid">
<div class="row-fluid">
<div class="span3">
<div class="well sidebar-nav">
<ul class="nav nav-list">
<li class="nav-header">Sidebar</li>
<li class="active"><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li class="nav-header">Sidebar</li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li class="nav-header">Sidebar</li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
</ul>
</div>
</div>
<div class="span6">
<div class="hero-unit">
<h1>Hello, world!</h1>
<p>This is a template for a simple marketing or informational website. It includes a large callout called the hero unit and three supporting pieces of content. Use it as a starting point to create something more unique.</p>
<p><a class="btn btn-primary btn-large">Learn more »</a></p>
</div>
<div class="row-fluid">
<div class="span4">
<h2>Heading</h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Etiam porta sem malesuada magna mollis euismod. Donec sed odio dui. </p>
<p><a class="btn" href="#">View details »</a></p>
</div>
<div class="span4">
<h2>Heading</h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Etiam porta sem malesuada magna mollis euismod. Donec sed odio dui. </p>
<p><a class="btn" href="#">View details »</a></p>
</div>
<div class="span4">
<h2>Heading</h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Etiam porta sem malesuada magna mollis euismod. Donec sed odio dui. </p>
<p><a class="btn" href="#">View details »</a></p>
</div>
</div>
<div class="row-fluid">
<div class="span4">
<h2>Heading</h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Etiam porta sem malesuada magna mollis euismod. Donec sed odio dui. </p>
<p><a class="btn" href="#">View details »</a></p>
</div>
<div class="span4">
<h2>Heading</h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Etiam porta sem malesuada magna mollis euismod. Donec sed odio dui. </p>
<p><a class="btn" href="#">View details »</a></p>
</div>
<div class="span4">
<h2>Heading</h2>
<p>Donec id elit non mi porta gravida at eget metus. Fusce dapibus, tellus ac cursus commodo, tortor mauris condimentum nibh, ut fermentum massa justo sit amet risus. Etiam porta sem malesuada magna mollis euismod. Donec sed odio dui. </p>
<p><a class="btn" href="#">View details »</a></p>
</div>
</div>
</div>
<div class="span3">
<div class="well sidebar-nav">
<ul class="nav nav-list">
<li class="nav-header">Sidebar</li>
<li class="active"><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li class="nav-header">Sidebar</li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li class="nav-header">Sidebar</li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
</ul>
</div>
</div>
</div>
<hr>
<footer>
<p>© Company 2012</p>
</footer>
</div>
Demo: http://jsfiddle.net/6vPqA/89/
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