Navigation - Level Limits
Your theme is able to create nested menus up to 3 levels deep - the Shopify admin area's limit - from a single page by using the menu editing interface.
Here is more information on how to do this:
Shopify Help - Drop-down Menus
There is also the option of using the mega menu feature which increases your layout options. More information is available on this towards the bottom of this page, along with a useful tutorial video:
(This video is for Envy, but the same setup process applies for Flow)
You also have the option of adding additional functionality, via the use of an app, to display additional layers of navigation such as: