Если Вам требуется вывод категорий третьего уровня в горизонтальном меню системы управления OPencart 2.3 без сторонних расширений, сделайте правки в дефолтном шаблоне Opencart:

код в файле /catalog/controller/common/header.php

$children_data[] = array(
						'name'  => $child['name'] . ($this->config->get('config_product_count') ? ' (' . $product_total . ')' : ''),
						'href'  => $this->url->link('product/category', 'path=' . $category['category_id'] . '_' . $child['category_id'])	
					);
}

меняем на

// Menu
$this->load->model('catalog/category');
$this->load->model('catalog/product');
 
$data['categories'] = array();
 
  $categories = $this->model_catalog_category->getCategories(0);
 
  foreach ($categories as $category) {
    if ($category['top']) {
 
   // Level 2
   $children_data = array();
   $children = $this->model_catalog_category->getCategories($category['category_id']);
 
   foreach ($children as $child) {
     $filter_data = array(
       'filter_category_id'  => $child['category_id'],
       'filter_sub_category' => true
     );
 
   //level 3
   $subchildren_data = array();
   $subchildren = $this->model_catalog_category->getCategories($child['category_id']);
 
   foreach ($subchildren as $subchild) {
     $filter_data = array(
       'filter_category_id'  => $subchild['category_id'],
       'filter_sub_category' => true
     );
 
   $subchildren_data[] = array(
     'name'  => $subchild['name'] . ($this->config->get('config_product_count') ? ' <span class="menu_summ">' . $this->model_catalog_product->getTotalProducts($filter_data) . '</span>' : ''),
     'href'  => $this->url->link('product/category', 'path=' . $category['category_id'] . '_' . $child['category_id']. '_' . $subchild['category_id']),
   );
}
 				
  $children_data[] = array(
    'name'        => $child['name'] . ($this->config->get('config_product_count') ? ' <span class="menu_summ">' . $this->model_catalog_product->getTotalProducts($filter_data) . '</span>' : ''),
    'href'        => $this->url->link('product/category', 'path=' . $category['category_id'] . '_' . $child['category_id']),
    'subchildren' => $subchildren_data
    );
}
 				 
 // Level 1
  $data['categories'][] = array(
    'name'     => $category['name'],
    'children' => $children_data,
    'column'   => $category['column'] ? $category['column'] : 1,
    'href'     => $this->url->link('product/category', 'path=' . $category['category_id'])
  );
 }
}

В файле /catalog/view/theme/default/template/common/header.tpl

после кода

<li><a href="/<?php echo $child['href']; ?>"><?php echo $child['name']; ?></a></li>

 добавить

<?php if($child['subchildren']) { ?>
  
    <ul class="list-unstyled">
     <?php foreach ($child['subchildren'] as $subchild) { ?>  
       <li><a class="pull-left" href="/<?php echo $subchild['href']; ?>"><?php echo $subchild['name']; ?></a></li>
     <?php } ?> 
    </ul>
 
 <?php } ?>