3차 카테고리를 만들려면 어떻게 해야하나요? 채택완료

아래의 폼에서

1차 카테고리 -> 서브카테고리(2 카테고리) -> 서브서브카테고리(3차카테고리)를 만들고 싶습니다.

감사합니다. 

<form class="filter_form" id="service_filter_form" name="service_filter_form" method="get">
    <div class="row">
        <div class="col-md-3">
            <select class="kb-select initialized" onchange="this.form.submit()" name="category" id="category">
                <option value="0"><?php echo translate('all') . " " . translate('category'); ?></option>
                <?php foreach ($service_category as $val): ?>
                    <option <?php echo ($category == $val['id']) ? "selected='selected'" : ""; ?> value="<?php echo $val['id'] ?>"><?php echo $val['title'] ?></option>
                <?php endforeach; ?>
            </select>
        </div>
        <div class="col-md-3">
            <select class="kb-select initialized" onchange="this.form.submit()" name="vendor" id="vendor">
                <option value="0"><?php echo translate('all') . " " . translate('vendor'); ?></option>
                <?php foreach ($vendor_data as $val): ?>
                    <option <?php echo ($vendor == $val['id']) ? "selected='selected'" : ""; ?> value="<?php echo $val['id'] ?>"><?php echo $val['company_name'] ?></option>
                <?php endforeach; ?>
            </select>
        </div>
        <div class="col-md-3">
            <select class="kb-select initialized" onchange="this.form.submit()" name="location" id="location">
                <option value="0"><?php echo translate('all') . " " . translate('location'); ?></option>
                <?php foreach ($app_location as $val): ?>
                    <option <?php echo ($location == $val['loc_id']) ? "selected='selected'" : ""; ?> value="<?php echo $val['loc_id'] ?>"><?php echo $val['loc_title'] ?></option>
                <?php endforeach; ?>
            </select>
        </div>
        <div class="col-md-2">
            <a class="btn btn-info" href="<?php echo base_url('services'); ?>"><i class="fa fa-refresh"></i></a>
        </div>
    </div>
</form>