[middlee]

Решил проблему, не так как хотелось конечно, но на первое время сойдет, выкладываю решению, может кому-то пригодится:

<select id="country" name="country">
        <option>Выберите страну...</option>
    {foreach $row_country as $country}
        <option {if $country.country_id == $smarty.get.country} selected {/if} value="{$country.country_id}">{$country.name}</option>
    {/foreach}
    </select>
    <select id="region" name="region">
        <option>Выберите регион...</option>
    {foreach $row_region as $region}
        <option {if $region.region_id == $smarty.get.region} selected {/if} value="{$region.region_id}">{$region.name}</option>
    {/foreach}
    </select>
    <select id="city" name="city">
        <option>Выберите город...</option>
    {foreach $row_city as $city}
        <option value="{$city.id}">{$city.name}</option>
    {/foreach}
    </select>

$('#country').change( function() {
    var country = $('#country').val();
    $.ajax({
		type: "GET",
		url: "index.php?option=registration&country="+country,
		data: country,
		success: function(data) {
            document.location.href = "index.php?option=registration&country="+country;
        }
	});
});
$('#region').change( function() {
    var region = $('#region').val();
    var country = $('#country').val();
    $.ajax({
		type: "GET",
		url: "index.php?option=registration&country="+country+"&region="+region,
		data: region,
		success: function(data) {
            document.location.href = "index.php?option=registration&country="+country+"&region="+region;
        }
	});
});

class Registration extends Core {
    public function getContent() {
        global $mysqli;
        global $smarty;
        
        $res = $mysqli->query("SELECT * FROM country ORDER BY name");
            if ($res->num_rows > 0) {
                for ($i=0; $i<$res->num_rows; $i++) {
                    $row_country[] = $mysqli->assoc($res);
                }
            $smarty->assign("row_country", $row_country);
            $smarty->assign("country", $_POST['country']);
            }
        
        $res_region = $mysqli->query("SELECT * FROM region WHERE country_id=".$_GET['country']." ORDER BY name");
            if ($res_region->num_rows > 0) {
                for ($j=0; $j<$res_region->num_rows; $j++) {
                    $row_region[] = $mysqli->assoc($res_region);
                }
            $smarty->assign("row_region", $row_region);
            }
        
        $res_city = $mysqli->query("SELECT * FROM city WHERE region_id=".$_GET['region']." ORDER BY name");
            if ($res_city->num_rows > 0) {
                for ($j=0; $j<$res_city->num_rows; $j++) {
                    $row_city[] = $mysqli->assoc($res_city);
                }
            $smarty->assign("row_city", $row_city);
            }
    }
}

laimas держи +, помог