<?php

$arr1 = array( array(
                    'letter' => 'A',
                    'number' => '1'
                ),

               array(
                    'letter' => 'B',
                    'number' => '2'
                ),

               array(
                    'letter' => 'C',
                    'number' => '3'
                )

    );

$arr2 = array( array(
                    'letter' => 'A',
                    'number' => '1'
                ),

               array(
                    'letter' => 'B',
                    'number' => '2'
                )

    );



 $arr3 = array( array(
                    'letter' => 'D',
                    'number' => '4'
                ),

               array(
                    'letter' => 'E',
                    'number' => '5'
                )

    );

$mergeArr = array_merge($arr1,$arr3);

$res = array_diff_assoc($mergeArr, $arr2);

var_dump($res);

array(3) {
  [2]=>
  array(2) {
    ["letter"]=>
    string(1) "C"
    ["number"]=>
    string(1) "3"
  }
  [3]=>
  array(2) {
    ["letter"]=>
    string(1) "D"
    ["number"]=>
    string(1) "4"
  }
  [4]=>
  array(2) {
    ["letter"]=>
    string(1) "E"
    ["number"]=>
    string(1) "5"
  }
}