你非常接近。您只想在保存评论的正确时间加入。这是未经测试的，但应该有效。

add_action( \'comment_post\', \'wpse_make_private_after_3_comments\', 10, 2 );

function wpse_make_private_after_3_comments( $comment_ID, $comment_approved ) {
    $comment = get_comment( $comment_ID );
    $post_ID = $comment->comment_post_ID;
    $comments = wp_count_comments( $post_ID );

    // You could also access approved, moderated, spam or trashed comments
    // from the return object of wp_count_comments().

    $comment_count = $comments->total_comments;

    // If we only have 1 or 2 comments, we\'ll bail early
    if ( $comment_count < 3 ) {
        return;
    }

    $post_data = array(
        \'ID\' => $post_ID,
        \'post_status\' => \'private\'
    );

    wp_update_post( $post_data );

    // You might want to add a wp_redirect() here to 
    // so people don\'t automatically see a 404 page 
    // when the comment saving is complete since the page will be private.
}