The following code shows how to mark column with span3 for well.
<!DOCTYPE html>
<html>
<head>
<script type='text/javascript'
src='/js/lib/mootools-core-1.4.5-nocompat.js'></script>
<style type='text/css'>
@import<!--from ww w .j a v a2s . c o m-->
url(http://netdna.bootstrapcdn.com/twitter-bootstrap/2.1.0/css/bootstrap-combined.min.css)
;
</style>
</head>
<body style='margin:30px'>
<div class="row">
<div class="span3 well">
<p>I'm really short</p>
</div>
<div class="span3 well">
<p>I'm in the middle</p>
<p>I'm in the middle</p>
<p>I'm in the middle</p>
</div>
<div class="span3 well">
<p>I'm really long</p>
<p>I'm really long</p>
<p>I'm really long</p>
<p>I'm really long</p>
<p>I'm really long</p>
<p>I'm really long</p>
</div>
</div>
</body>
</html>