<?php
$categories = get_categories('child_of=3');//child_ofは出力内容に変更
foreach($categories as $category) :
?>
<?php
query_posts('cat='.$category->cat_ID);
if (have_posts()) : while (have_posts()) : the_post();
?>
表示内容
<?php endwhile; endif; ?>
<?php endforeach; ?>