function categoryLoop($id, $is_sub = false){
    $objectManager = MagentoFrameworkAppObjectManager::getInstance();

$categories = $objectManager->create('MagentoCatalogModelCategory')->load($id);
if($categories->hasChildren()){
echo '<ul class="leftcat"' . (($is_sub) ? ' class="category_children category-'.$categories->getId().'"' : '') . '>';
        $subcategories = explode(',', $categories->getChildren());
        foreach ($subcategories as $category) {
            $subcategory = $objectManager->create('MagentoCatalogModelCategory')->load($category);
              echo '<li><a href="'.$subcategory->getUrl().'">';
            echo $subcategory->getName();
            echo "</a></li>";
            if($subcategory->hasChildren()){ categoryLoop($category, true); }
        }
    echo "</ul>";
    }
}
categoryLoop(3);