Using switch statement with 3 dynamic dropdowns

I'm using this example True Tech Troubleshooting: Creating Dynamic Dropdowns Using JavaScript in Adobe LiveCycle Designer ES 4® which is spot on, however I wanted to add in one more switch statement. Using this example I would like for the year to switch the make then the make switch the model.

Example 1930s only shows cars made in 1930, then I select the make which provides the models. How do I achieve adding a switch statement to the year.

I have some JS skills but not implementing them in Livecycle designer in this manner.