2 criteria for dependent drop down list

Hi all- I was wondering if its possible to create three separate drop downs using data validation. The first 2 are independent of each other, and the third is dependent on the combination in the first 2 dropwdowns. For example:

First drop down: provides a list of counties
Second drop down: provides a list of work classes
Third drop down: provides a list of trades based on the selections made in the First and Second drop down.

Does that make sense? quite urgent, but any help will be greatly appreciated.

plz attach your input file