How can i add sub child in child item using expandable-recycler-view?

Question

I am using thoughtbot's expandable-recycler-view library and i have successfully integrated expandable-recycler-view. i have a question how can i make child view expandable?

Like below.

Item 1  ↓
      Sub Item 1  ↑
      Sub Item 2  ↑
      Sub Item 3  ↓
              Cat 1
              Cat 2
              Cat 3
     Sub Item 4  ↑
Item 2  ↑
Item 3  ↑
Item 4  ↑

Right now, I can add only One layer child in Adapter.

public class CottonAdapter
        extends
        ExpandableRecyclerAdapter<Reslatestprice.Report,Reslatestprice.Product,
                CottonAdapter.ViewHolderParent, CottonAdapter.ViewHolderChild>