如何获取magento中的所有3个级别的类别？

I want to show all top level categories and its respective 2 level of subcategories. for example Root category(id=3)

-Electronics
  --Computers
   ---Dell
   ---Samsung
   ---Accer

 -Games Movies and Musics
  --Games
   ---Pc Games
   ---Mobile Games

Here Electronics and Games Movies and Musics is top level categories I don't want to display root category, only from top level categories i.e from electronics and Games Movies and Musics

Like this

$category_id = 'Electronics category id'
$children = Mage::getModel('catalog/category')->getCategories($category_id);
foreach ($children as $category) {
echo $category->getName();
}

Update: You can keep looping through them like this

$category_id = 'Electronics category id';
    $children = Mage::getModel('catalog/category')->getCategories($category_id);
    foreach ($children as $category) {
        echo '<h1>'.$category->getName().'</h1>';
        if($category->getChildren()){
            $childrensChildren = $category->getChildren();
            foreach($childrensChildren as $ChildCategory){
                echo '<h2>'.$ChildCategory->getName().'</h2>';
            }
        }

    }