Finding split points in an article for inserting ads

Hey all - I've got long articles, the HTML for which comes out of MySQL. Works great. I want to split it up so that I can insert ad blocks at various points within it. The articles are all pretty long but they're of variable length. I want to chop them up into three close-to-equal (doesn't have to be) chunks of paragraphs, thus: <p>Here's the original article</p> <p>Here's the original article</p> <p>Here's the original article</p> <p>Here's the original article</p> <p>Here's the original article</p> Would become: <p>Here's the original article</p> <p>Here's the original article</p> <?php include('first_ad'); ?> <p>Here's the original article</p> <p>Here's the original article</p> <?php include('second_ad'); ?> <p>Here's the original article</p> Any suggestions? Thanks.