With the latest bootstrap nav module (V1.4 and on) the option of a mega-menu has been introduced. First, you set the menu width's to your liking. there are 2. The submenu width, and the mega-menu width. Once done, you can start publishing sub-menus. Simply add the term 'mega-menu' in the link-title attribute, and this menu becomes a mega-menu.
In the bootstrap nav settings, you can then set up the number of columns, the width, and the divider color
Check this website's extensions menu. That is an example of a mega menu.
Note: third level mega menus will not render well, and usability also advises not to use 3rd level mega menus.