I would do it a different way and not use an acf have_rows() loop
$repeater = get_field('testimonialREPEATER', 1480);
$indexes = array_rand($repeater, 3);
foreach ($indexes as $index) {
var_dump($repeater[$index]);
}
Using the above, instead of using the_sub_field() you would need to access the value in the array, for example testimonialQUOTE
would be
echo $repeater[$index]['testimonialQUOTE'];