WP Query / query_posts how to use OR condition

Hi,

I can’t find, how to select one post from category_name x OR category_name y OR category_name z

Can someone help me please?

I tried this:

$args = array(
            'posts_per_page' => 1,
            'category_name' => array(
                $prefix."sun", $prefix."water"
            )
        );
        $the_query = new WP_Query( $args );

And this:

$args = array(
            'posts_per_page' => 1,
            'category_name__in' => array($prefix."sun", $prefix."water")
        );
        $the_query = new WP_Query( $args );

It doesn’t work

Thank you