我能想到的唯一方法就是使用save_post action hook. 因为你的文章不是通过AJAX保存的，而且只有在你真正保存文章的时候，我认为那才是最好的地方。

老实说，我还没有写过很多错误消息，所以这可能是不可靠的，可能在一开始就行不通，但我从this article from SitePoint.

add_action( \'save_post\', \'my_save_post_function\' );
function my_save_function( $post_id ) {
    $error = false;

    $featured = get_posts(array(
        \'meta_query\' => array(
            array(
                \'key\' => \'featured_article\',
                \'compare\' => \'=\',
                \'value\' => \'1\'
            )
        )
    ));

    if ( count($featured) > 3 ) {
        $error = new WP_Error(\'too_many_featured_articles\', \'There are too many featured articles.\');
        // You could use this error to list the titles and links to edit all of the other current featured articles.
    }

    if ($error) {
        // Here is where you would unset the ACF field, if you wanted.
    }

    return true;
}