Sub sub-items

is there a way to add sub items to sub items?

I believe there are some third party apps that can accomplish this, but natively within Monday there is not technically a layer below subitems.

What our team does is uses checklists in the updates of subitems to act as a layer below sub-items. If you want to learn more about checklists here is an article from Monday - this option is great if you are looking for something that does not cost extra and is natively within Monday, the third party apps will likely have a cost associated with them, but might offer some additional functionality.