$someheader = 'this-page-header'; // Or some logic that sets $someheader based on the page
A less graceful way to go about it (as far as programming) - but would give you more control from the admin interface - is to create a custom field that sets what header to use. If you don't set it, it uses the default header.
So let's say in custom fields you create a field 'custom-header' and set the value 'third-page'. Wordpress looks for files that begin with 'header', so this would turn into 'header-third-page'.