Hi @mariepell,
I hope you are well today and thank you for your question.
This theme is built on Bootstrap and by default it doesn’t support third level of menu structure and one submenu is max so to achieve this you have to develop custom code by referring the solution posted in the following topics.
https://colorlibsupport.com/t/how-do-you-display-three-level-menus-in-unite-theme/
https://colorlibsupport.com/t/child-theme-menu-override-in-functions-php-adding-a-third-level-of-menu-depth/
Best Regards,
Movin