Populating Drop Down Lists from Drop Down Selections

Hi there

I hope some can help, I am a relative novice in scripting and am building a form that has 3 dropdown lists dependent on one another.

List 1 are Departments

List 2 are Functions (within those departments and only should display when the department is selected in List 1)

List 3 are Jobs (within those functions and should only display when the function is selected in List 2)

I am have written a switch statement for functions to display once the department (as the departments have values in the list) has been selected, but am struggling to find the right script to populate the drop down with jobs from the function selection. Can someone please assist

Any support would be very much appeciated