How to query over the network

I have a WP 3.8.1 Multisite installation with 3 blogs, they’re all in one db, prefixed wp_, wp_2_ and wp_3_.

In a template in wp_ I try to to display all posts from wp_, wp_2_ and wp_3_ which have a category xxx attached. I want to display the posts chronologically. I tried this query:

SELECT *
FROM $wpdb->posts
    	INNER JOIN $wpdb->term_relationships ON $wpdb->posts.ID= $wpdb->term_relationships.object_id
    	INNER JOIN $wpdb->term_taxonomy ON $wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id
    	INNER JOIN $wpdb->terms ON $wpdb->term_taxonomy.term_id=$wpdb->terms.term_id WHERE $wpdb->terms.name = 'xxx'
    ORDER BY $wpdb->posts.post_date DESC

This works only for the current blog, ie it doesn’t include wp_2_ and wp_3_.

How do I retrieve all posts with category xxx from wp_, wp_2_, wp_3_ (instead of only wp_)?