<pre>
<?
require_once($_SERVER['DOCUMENT_ROOT'] . "/bitrix/modules/main/include/prolog_before.php");
CModule::IncludeModule('iblock');

ini_set("memory_limit", "4096M");
set_time_limit(100000);

// Функция поиска товара по свойству
function searchElementArr($value, $array) // ("ЗНАЧЕНИЕ_ID", "МАССИВ_ГДЕ_ИЩЕМ")
{
    foreach ($array as $i => $element) {
        if (trim($element["ID"]) === $value) {
            return $element["ID"];
        }
    }
}

$arr = array(

);

$arr1 = array (
    "0" => array(
        "art" => "546436456",
    ),
    "1" => array(
        "art" => "7683435",
    ),
);

$arr2 = array (
    "0" => array(
        "art" => "546436456",
    ),
    "1" => array(
        "art" => "76653834543283435",
    ),
);

$arr3 = array (
    "0" => array(
        "art" => "546436456",
    ),
    "1" => array(
        "art" => "45435233",
    ),
);

echo "<b>Arr1</b> | <br>";
foreach ($arr1 as $i => $el) {
    if (empty(searchElementArr($el["art"], $arr))) {
        array_push($arr, array(
            "ID" => $el["art"],
        ));
        echo "<br> Товар добавлен [art] => ".$el["art"]." <br>";
    } else { echo "<br> Товар изменён <br>"; }
}

echo "<br> <b>Arr2</b> | <br>";
foreach ($arr2 as $i => $el) {
    if (empty(searchElementArr($el["art"], $arr))) {
        array_push($arr, array(
            "ID" => $el["art"],
        ));
        echo "<br> Товар добавлен [art] => ".$el["art"]." <br>";
    } else { echo "<br> Товар изменён [art] => ".$el["art"]." <br>"; }
}

echo "<br> <b>Arr3</b> | <br>";
foreach ($arr3 as $i => $el) {
    if (empty(searchElementArr($el["art"], $arr))) {
        array_push($arr, array(
            "ID" => $el["art"],
        ));
        echo "<br> Товар добавлен [art] => ".$el["art"]." <br>";
    } else { echo "<br> Товар изменён [art] => ".$el["art"]." <br>"; }
}

echo "<br> <b>Итоговый масив Arr</b> | <br>";
print_r($arr);

?>