Splitting Content field in post into three fields

I need to break the one, big “Post” field in the post/page form into three fields. I have successfully created the three fields, all with the WYSIWYG editor, but the javascript I am using to concantenate the three fields and submit as “content” isn’t working.

I try to access the form with “document.forms.post.<?php echo ($id); ?>.value = $field + $field;. This script works on pages I created on my own. But it does not seem to access the post form.

Help.