What I want to do is use a drop-list with three(3) options. Based on the
option picked from the first drop-list, the other drop-lists will only
display the data relevant to the selected option. This way in the subsequent
drop-lists will only display the correct information. For example.

Main drop-list has three options: 1. Mild Steel, 2. Stainless Steel, 3.
Aluminum

Second drop-list will only have the thicknesses for Mild
Third will only have thicknesses for Stainless and so on.