Why is the third level not showing in my superfish menu?
I would give this a try:


  In the drupal bloc configuration just disable "Slide-in effect"


Quoting the last comment from this thread:
https://github.com/joeldbirch/superfish/issues/30

Hope you get lucky with this, that whole github thread could be also interesting/relevant to your issue.

Update: This is apparently a theme specific superfish implementation, the respective theme being (most probably) Nexus

In order to deactive the effects that could be causing the error, you should edit the file sites/all/themes/nexus/js/custom.js at about line 7, commenting out the animation options:

//  animation:   { opacity:'show',height:'show'},   // fade-in and slide-down animation 


Hope this does the trick!