codice:
<?php
$arr = array(
array(3, '300€', 1, 1, 842),
array(3, '300€', 1, 2, 842),
array(3, '300€', 1, 3, 842),
array(3, '300€', 1, 2, 843),
array(3, '300€', 1, 1, 843),
array(4, '300€', 1, 1, 843),
array(3, '300€', 1, 3, 843)
);

$unique     = array();
$duplicates = array();

foreach ($arr as $item) {
	$item_slice = array_slice($item, 0, 3);
	if (!in_array($item_slice, $unique)) {
		$unique[] = $item_slice;
	} else {
		$duplicates[] = $item;
	}
}

print_r($unique);     // array con valori unici
print_r($duplicates); // array con i duplicati rimossi