Append custom taxonomy to CPT

rand_cat_to_post.php

<?php
$tax = 'YOUR_TAX_HERE';
$cpt = 'YOUR_CPT_HERE';
$count = 3;
$types = get_terms(array(
	'taxonomy'   => $tax,
	'hide_empty' => false,
	'parent'     => 0
));
$arr = array(
	'post_type' => $cpt,
	'posts_per_page' => -1
);
$func = function($value) {
	return $value->term_id;
};
$ids = array_map($func, $types);

$q = new WP_Query($arr);
while($q->have_posts()) : $q->the_post();
	global $post;
	$randIndex = array_rand($ids, 3);

	$randomTax = array();
	foreach($randIndex as $i) {
		$randomTax[] = $ids[$i];
	}
	wp_set_post_terms($post->ID, $randomTax, $tax, false );
endwhile;

wp_reset_query();