Hello there,
I hope you are doing well today.
The third level menu system is not a feature of the theme because it can be unstable with WordPress updates however someone has created a work around for the Sparkling theme which could be used here.
Have a look:
https://colorlibsupport.com/t/child-theme-menu-override-in-functions-php-adding-a-third-level-of-menu-depth/
Best Regards,
Support