create local copy of this modal:
app\code\local\Mage\Catalog\Model\Resource\Eav\Mysql4\Product
edit collection.php  add this function:-
public function addCategoriesFilter(array $categories)
    {
       // print_r($categories);
  //echo $this->getStoreId() ;  
        $this->_prepareProductLimitationFilters();
        $this->_productLimitationJoinWebsite();
        $this->_productLimitationJoinPrice();
        $filters = $this->_productLimitationFilters;
        $conditions = array(
            'cat_index.product_id=e.entity_id',
            $this->getConnection()->quoteInto('cat_index.store_id=?', $this->getStoreId())
        );
        if (isset($filters['visibility']) && !isset($filters['store_table'])) {
            $conditions[] = $this->getConnection()
                ->quoteInto('cat_index.visibility IN(?)', $filters['visibility']);
        }
  $conditions[] = $this->getConnection()
            ->quoteInto('cat_index.category_id IN(?)',$categories);
        if (isset($filters['category_is_anchor'])) {
            $conditions[] = $this->getConnection()
                ->quoteInto('cat_index.is_parent=?', $filters['category_is_anchor']);
        }
        $joinCond = join(' AND ', $conditions);
                $this->getSelect()->join(
                array('cat_index' => $this->getTable('catalog/category_product_index')),
                $joinCond,
                array('cat_index_position' => 'position')
            );    
        $this->_productLimitationJoinStore();
        Mage::dispatchEvent('catalog_product_collection_apply_limitations_after', array(
            'collection'    => $this
        ));
        return $this;
    }
At your module block use this function to get products collection from multiple categories :-
public function _getProductCollection(){
  $visibility = array(
  Mage_Catalog_Model_Product_Visibility::VISIBILITY_BOTH,
  Mage_Catalog_Model_Product_Visibility::VISIBILITY_IN_CATALOG
 );
 $categories=array(3,4,5,6,7,8);
 $storeId    = Mage::app()->getStore()->getId();
 $this->_collection = Mage::getModel('catalog/resource_eav_mysql4_product_collection')
 ->setStoreId($storeId)
 ->addCategoriesFilter($categories)
 ->addAttributeToFilter('visibility', $visibility)
 ->addAttributeToSelect('*')
 ->setOrder('created_at', 'desc')
    ->addAttributeToSelect('*');
 return $this->_collection; 
 }