Retrieve featured image 3

Hi, I’m editing the grid builder of visual composer to show one of the featured images uploaded with your plugin, the featured image 3.

I have upload correctly featured images 3 and then I have added the filter as the visual composer documentation shows:

add_filter( 'vc_grid_item_shortcodes', 'my_module_add_grid_shortcodes' );
function my_module_add_grid_shortcodes( $shortcodes ) {
   $shortcodes['kdmfi_the_featured_image'] = array(
        'name' => __( 'Logo negozio', 'js_composer' ),
        'base' => 'kdmfi_the_featured_image',
        'icon' => 'vc_icon-vc-gitem-image',
        'category' => __( 'Content', 'js_composer' ),
        'description' => __( 'Logo negozio', 'js_composer' ),
        'post_type' => Vc_Grid_Item_Editor::postType('negozi-millennium'),
    );
    return $shortcodes;
}
 
add_shortcode( 'kdmfi_the_featured_image', 'kdmfi_the_featured_image_render' );
function kdmfi_the_featured_image_render() {
   return kdmfi_the_featured_image('featured-image-3' , 'full');
}

But the featured image 3 is not visible.

My problem is in the final function add_shortcode: in the final return what do I have to insert to recall the featured images 3?

I hope you can help me, thanks very much.

Alex