The function can be placed into your functions.php, and the "usage" snippet can be used in any template/theme file.

related.php

<?php

/**
 * Returns a WP_Query of downloads that are in the same download category,
 * or download tag, order by latest, limited to only 3
 *
 * @since 1.0.0
 *
 */
function my_edd_related_taxonomies_query(){

    $cat_taxonomy = 'download_category';
    $download_cat_obj = get_the_terms( $post->ID, $cat_taxonomy );
    $download_cat_ids = array();

    if ( $download_cat_obj && ! is_wp_error( $download_cat_obj ) ) {
        foreach( $download_cat_obj as $download_category_obj ){
            $download_cat_ids[] = $download_category_obj->term_id;
        }
    }

    $tag_taxonomy = 'download_tag';
    $download_tag_obj = get_the_terms( $post->ID, $tag_taxonomy );
    $download_tag_ids = array();

    if ( $download_tag_obj && ! is_wp_error( $download_tag_obj ) ) {
        foreach( $download_tag_obj as $download_category_obj ){
            $download_tag_ids[] = $download_category_obj->term_id;
        }
    }

    global $post;

    $the_query = New WP_Query( array(
            'post_type' => 'download',
            'posts_per_page' => 3,
            'post_status' => 'publish',
            'post__not_in' => array( $post->ID ),
            'tax_query' => array(
                'relation' => 'OR',
                array(
                    'taxonomy' => $cat_taxonomy,
                    'field' => 'term_id',
                    'terms' => $download_cat_ids
                    ),
                array(
                    'taxonomy' => $tag_taxonomy,
                    'field' => 'term_id',
                    'terms' => $download_tag_ids
                    )
                )
    ) );

    return $the_query;

}


// Usage, as in any "loop"
$related_category_query = my_edd_related_taxonomies_query();
if ( $related_category_query->have_posts() ) : ?>
    <?php while ( $related_category_query->have_posts() ) : $related_category_query->the_post(); ?>
        <?php the_title(); ?>
    <?php endwhile; ?>
<?php endif; ?>