The theme currently supports two levels in the navigation, so a third level will not display. However I think a good solution is to use a dash or bullet point to denote a third level, like so:

To get this effect, you can just edit the title section of the navigation links under Appearance > Menus and add in the symbol:

It's not a full fledged third level, but one benefit to this method is that users are much less likely to accidentally close the dropdown as can happen with multiple levels of flyout menus :)