You can use the switch_to_blog() function to switch the scope to a specific site and access data from that site:
require_once( '/path/to/wp-load.php' );
switch_to_blog( 3 );
$option = get_option( 'admin_email' );
restore_current_blog();
Also, you should use wp-load.php when using WordPress API in your own project, not wp-blog-header.php