query_postsで複数条件の指定

=から右をカンマ区切りで指定すれば出来ると思います。
query_posts(‘meta_key=test,test2,test3');

自分もWordPressに触れて日が浅いため間違っていたらすみません
参考：WordPres Codex テンプレートタグ/query posts

未検証なので細かいことはよきにはからってください。
functions.php:

function my_posts_where($where){
  global $wpdb;
  return $where . " AND $wpdb->postmeta.meta_key IN ('test1','test2','test3') ";
}
function my_posts_join($join){
  global $wpdb;
  return $join . " JOIN $wpdb->postmeta ON ($wpdb->posts.ID = $wpdb->postmeta.post_id) ";
}
function my_query_posts($q=''){
  add_filter('posts_where', 'my_posts_where');
  add_filter('posts_join', 'my_posts_join');
  query_posts($q);
  remove_filter('posts_where', 'my_posts_where');
  remove_filter('posts_join', 'my_posts_join');
}

で query_posts() の代わりに my_query_posts() を使う。か、

global $wpdb;
$posts = $wpdb->get_results("
SELECT $wpdb->posts.*
 FROM $wpdb->posts
 JOIN $wpdb->postmeta ON ($wpdb->posts.ID = $wpdb->postmeta.post_id)
 WHERE $wpdb->postmeta.meta_key IN ('test1','test2','test3')
 ORDER BY $wpdb->posts.post_date DESC
");
foreach($posts as $post) :
  /* do stuff */
endforeach;

これも未検証なのでよろしくです。

この重複を避けるには

functions.php に追加＆変更：

function my_posts_distinct($distinct){
  return 'DISTINCT';
}
function my_query_posts($q=''){
  add_filter('posts_distinct', 'my_posts_distinct');
  add_filter('posts_where', 'my_posts_where');
  add_filter('posts_join', 'my_posts_join');
  query_posts($q);
  remove_filter('posts_distinct', 'my_posts_distinct');
  remove_filter('posts_where', 'my_posts_where');
  remove_filter('posts_join', 'my_posts_join');
}

または
SELECT DISTINCT $wpdb->posts.*

カスタムフィールドの”キーと値”の両方で複数判

functions.php 変更：
※全部一致の場合

function my_posts_where($where){
  global $wpdb;
  return $where . "
    AND $wpdb->postmeta.meta_key = 'test1'
    AND $wpdb->postmeta.meta_value = '東京'
    AND $wpdb->postmeta.meta_key = 'test2'
    AND $wpdb->postmeta.meta_value = '埼玉'
    AND $wpdb->postmeta.meta_key = 'test3'
    AND $wpdb->postmeta.meta_value = '神奈川'
  ";
}

※どれか一致の場合

function my_posts_where($where){
  global $wpdb;
  return $where . "
    AND (
      ( $wpdb->postmeta.meta_key = 'test1'
        AND $wpdb->postmeta.meta_value = '東京' )
      OR
      ( $wpdb->postmeta.meta_key = 'test2'
        AND $wpdb->postmeta.meta_value = '埼玉' )
      OR
      ( $wpdb->postmeta.meta_key = 'test3'
        AND $wpdb->postmeta.meta_value = '神奈川' )
    ) ";
}

または
WHERE $wpdb->postmeta.meta_key IN ('test1','test2','test3')
を上記に倣って変更。

Now my code is like this, works fine, but not quate accurate, I need to compare 2 meta_keys

function filter_where($where = '') {
	$date_start = ('30.03.2010');
	$date_end = ('30.04.2012');

    $where .= '
	AND STR_TO_DATE(meta_value,"%d.%m.%Y") >= STR_TO_DATE("'.$date_start.'","%d.%m.%Y")
	AND STR_TO_DATE(meta_value,"%d.%m.%Y") <= STR_TO_DATE("'.$date_end.'","%d.%m.%Y")
	';
    return $where;
}

function date_order($orderby = '') {
    $order .= 'STR_TO_DATE(meta_value,"%d.%m.%Y") ASC';
    return $order;
}

add_filter('posts_where', 'filter_where');
add_filter('posts_orderby', 'date_order');

$args = array(
	'category__and' => array($child_cat_id,$child_cat_id),
	'meta_key' => $meta_field_name,
	'showposts' => -1,
	);

$my_query = new WP_Query($args);

remove_filter('posts_where', 'filter_where');
remove_filter('posts_orderby', 'date_order');

Just in case =)

I think I got it. Thank you, will try soon.