Your code should actually look like this:
HTML Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
#
<jdoc:include type="head" />
#
<link rel="stylesheet" href="templates/<?php echo $this->template ?>/css/template.css" type="text/css" />
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Fixed Width CSS Layouts - 3 Column - fw-34-3-col</title>
<link rel="stylesheet" type="text/css" href="main.css" />
<?php
$template_baseurl = $this->baseurl . '/templates/' . $this->template;
// add template css to JDocumentHTML
$this->addStyleSheet($template_baseurl . '/template_css.css');
?>
</head>
<body>
You can use that code whenever you need to include additional CSS files but you should also use it to include your main CSS file. This would be your main.css file from your code above.