Different Subdomain changes page content

Just check the current host:

add_filter( 'the_content', 'domain_dependent_content' );

function domain_dependent_content( $content )
{
    // get_the_ID() will return the current post ID if you need 
    // more information about the current page.

    if ( 'page' !== get_post_type() )
        return $content;

    if ( 'au.example.com' === $_SERVER['HTTP_HOST'] )
        return "G'Day buddy!<br>$content";

    if ( 'uk.example.com' === $_SERVER['HTTP_HOST'] )
        return "How do you do?<br>$content";

    if ( 'us.example.com' === $_SERVER['HTTP_HOST'] )
        return "Hi!<br>$content";

    return $content;
}

You could also create metaboxes for the extra content, so the client can edit it. Then you check if there is extra content for this post with get_post_meta(), and if you found something, you change the content.