Loading a metabox for the correct post-type
Well, actually there's no point to call add_meta_box multiple times, because at the time when add_meta_box are called, the post type is passed by add_meta_boxes hook. (-amabil-)
Here is an example:
add_action( 'add_meta_boxes', 'my_add_custom_box' );
function my_add_custom_box($postType) {
$types = array('type1', 'type2', 'type3');
if(in_array($postType, $types)){
add_meta_box(
'myid',
__( 'Title', 'myplugin_textdomain' ),
'callback',
$postType
);
}
}