register_sidebars(1,
    array(
        'name' => 'Sidebar 3',
        'before_widget' => '<li class="widget %2$s" id="%1$s">',
        'after_widget' => '</li>',
        'before_title' => '<h3>',
        'after_title' => '</h3>'
    )
);
function extra_sidebar() { ?>
<div id="sidebar_3" class="sidebar">
<ul class="sidebar_list">
<?php thesis_default_widget(3); ?>
</ul>
</div>
<?php }
add_action('thesis_hook_after_sidebars','extra_sidebar');