Dash Cascading dropdown example - This was built out of looking at - Dash App With Chained Callbacks [https://dash.plotly.com/basic-callbacks]
My problem was I had 3 dependant dropdown list boxes Level1, Level2 and level 3
Level 3 contents depended on Level 2, Level 2 depended on Level 1 Level 1 had 3 choices and that was all I had to start with.
Hopoefully someone may find this useful.
I am just learning dash so at least I have it now.
This was created and tested with Python 3.8.3