WordPress Pages into Sections edit.php PHP hack

I need this to be done with PHP so that the user cannot set the Parent.

I asked this over on WP.SE and got a promising answer: Creating new page with pre-defined parent page I haven’t got around to testing it yet, though, as the project is on hold.

And I need to know how to divide up the pages into the relevant sections.

I’m not sure what you mean by that and how deep a division you need in your project. To display the three sections, you could use wp_list_pages() three times, and use the child_of parameter to pick the right section. Or you could have three menus in the backend, and call each of them using wp_nav_menu().