Filtering a drop down list based on first item

I have a form which has 3 items that the user can select. However, the first item is a department field and the way I have my form setup currently is the user can pick any random field and it will auto fill the other two fields. So the user has the ability to back-fill the form. However, the items in the form and not being filtered based on department if a user starts to fill out the form from the beginning and chooses a departments, its showing all the items for both the departments. Ideas on how I can only show the user the items for the specific department that they selected and still allow the user to back fill the form.

Not possible.