if (is_numeric($n = $_SERVER['QUERY_STRING'])) getGroup($n);
print_r(getGroup($n));
function getGroup($n) {$a=floor($n/3);$b=fmod($n,3);switch ($b) {case 0:return array('3'=>$a,'2'=>0);break;case 1:return array('3'=>$a-1,'2'=>2);break;case 2:return array('3'=>$a,'2'=>1);break;}}