It should read two files, it's a miss-print in the documents from a time when there was three files along time ago.
Simply you change the header and footer files to place the forum in a table around your design.
For example the header file starts the table and the footer file closes it, this is how this site does it using multiple tables.
The simplest way to do it is make a template of your page with a space where you want the forum to be, then place the part before the forum in the header and the part after in the footer.