Nested lists: 3rd field reliant on 2nd AND 1st

3 lists/records in hierarchical order:

Field Name - List/Record Values
Device type - Computer, Phone
Model - Dell Laptop, iPhone, Nokia 1100
Description - Big, Small, Expensive, Cheap

Select Computer in Device Type to see Dell Laptop, iPhone in Model
Select Phone in Device Type to see iPhone, Nokia 1100 in Model

With Computer in Device Type, select iPhone in Model to see Small, Expensive in Description
With Phone in Device Type, select iPhone in Model to see Expensive (since in the phone world it's no longer that small... for the sake of this example)


The problem is I can't figure out how to make iPhone in Model reveal different values in Description that are dependent also on the Device Type. Right now, I can do nested lists with 4 total fields, but there are overlaps. The top 3 of the 4 fields (heirarchy) source from custom records while the bottom is just a list.