The markup is probably there for the other dropdowns, just hidden. I recently did a nav menu with sub menus (they don’t create an extra dropdown) so I know it works.
This is the old nav.php file from about 8 months ago, before bootstrap 3, when bootstrap 2 supported the sub-sub menus: https://github.com/roots/roots/blob/ff6c28737156fb7ab32ec6d37b4345532b5dcf0c/lib/nav.php
This example on Bootply might help you: http://www.bootply.com/121468
All this said, be careful about your subnavs. They were removed from Bootstrap 3 for a reason - they’re less intuitive, and sometimes difficult to use.
Good luck!