Group by

gistfile1.php

<?php 

$usersGrouped = array();

foreach($users as $user) 
{
    $key = sprintf("%s:%s:%s", $user->field1, $user->field2, $user->field3);
    if(!isset($usersGrouped[$key]))
    {
        $usersGrouped[$key] = array();
    } 
    
    $usersGrouped[$key][] = $user;
}