<?php

$arr3_result= array(
array( 3 , array (6, 7, 8 )),
array( 6 , array (5, 6, 8 )),
array( 15 , array (5, 12, 13 )),
array( 16 , array (2, 3, 21, 22 )),
array( 1 , array ( 9, 15 )),
array( 6 , array ( 7 ))
);
asort($arr3_result);
$arr3_result=array_values($arr3_result);
foreach($arr3_result as $m=>$a){
	foreach($arr3_result as $n=>$b){
	            if($b[0]===$a[0] && $n>$m){
	                        $arr3_result[$m][1]=array_merge($a[1],$b[1]);
	                        unset($arr3_result[$n]);
	                        asort($arr3_result[$m][1]);
	            }
	}
	
}

print_r($arr3_result);