http://myhost.com/category-name/brand-name/product-name

class Vendor_Module_Model_Catalog_Product_Url extends Mage_Catalog_Model_Product_Url
{
    /**
     * Format Key for URL
     *
     * @param string $str
     * @return string
     */
    public function formatUrlKey($str)
    {
        // added '/' character
        $urlKey = preg_replace('#[^0-9a-z/.]+#i', '-', Mage::helper('catalog/product_url')->format($str));
        $urlKey = strtolower($urlKey);
        $urlKey = trim($urlKey, '-');

        return $urlKey;
    }

}

class Vendor_Module_Model_Catalog_Product_Attribute_Backend_Urlkey
    extends Mage_Catalog_Model_Product_Attribute_Backend_Urlkey
{
    /**
     * Before save
     *
     * @param Varien_Object $object
     * @return Mage_Catalog_Model_Resource_Product_Attribute_Backend_Urlkey
     * @overridden
     */
    public function beforeSave($object)
    {
        $attributeName = $this->getAttribute()->getName();

        $urlKey = $object->getData($attributeName);
        if ($urlKey == '') {
            $urlKey = "{$object->getBrandName()}/{$object->getName()}";
        }
        $object->setData($attributeName, $object->formatUrlKey($urlKey));

        return $this;
    }

}