Joy
(@joyously)
Ask in your theme support forum, because each theme does the menu in a different way.
From a design and UX viewpoint, three level dropdowns are practically unusable. For a client with a similar desire, we used a “mega menu” plugin that let us present the menus in a more 2 level way while still preserving the hierarchy.
https://wordpress.org/plugins/search/megamenu/
We used a premium plugin because there were a few additional needed features:
https://wpmegamenu.com/
Hover over “megamenu” on that page’s menu to get an idea of what I mean about the menu layout.