Three level navigation in Jekyll

I am trying to build a three level navigation on my sidebar menu. Currently the site only has two level list navigation. My yml file is at
The file with the relevant liquid code is at
Please could anyone advise on how I could modify these to enable three level navigation. This is referred to at, but I was unable to apply this advice to my site.

did you get it to work? it kind of looked like you did but I am not so sure.

is the 3rd level in your yml? could you maybe do a simpler example and post that?