//PLACE THIS IN YOUR FUNCTIONS.PHP FILE
<?php
$publishers = array     "pub-3152785879162963",
                        "id2",
                        "id3",
                        "id4",
                        "id5",
                        "id6",
                        "id7");

$randompublisher = array_rand($publishers,3);
?>

//ADD ONE LINES WHERE YOU WANT A RANDOM PUBLISHER ID.
//EACH LINE OUTPUTS AN UNIQUE ID FROM THE ARRAY YOUVE PLACED IN FUNCTIONS.PHP

<?php global $randompublisher; echo $publishers[$randompublisher[0]]; ?>
<?php echo $publishers[$randompublisher[1]]; ?>
<?php echo $publishers[$randompublisher[2]]; ?>