Three level navigation in Jekyll

#1

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 https://github.com/nhsconnect/FHIR-Maternity-Record/blob/gh-pages/_data/sidebars/overview_sidebar.yml
The file with the relevant liquid code is at https://github.com/nhsconnect/FHIR-Maternity-Record/blob/gh-pages/_includes/sidebar.html
Please could anyone advise on how I could modify these to enable three level navigation. This is referred to at https://jekyllrb.com/tutorials/navigation/#scenario-4-three-level-navigation-list, but I was unable to apply this advice to my site.

#2

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?