<?php
$iIds=array();
$iIds[0] = 12; 
$iIds[1] = 24;  
$iIds[2] = 25;  
$iIds[3] = 25;  
$iIds[4] = 25;  
$iIds[5] = 30;

$d = array_keys(array_filter( array_count_values($iIds), create_function('$x', 'return $x >= 3;')));
var_dump($d);
?>

array(1) {
  [0]=>
  int(25)
}