Dependent Drop Down Lists

This seems as though it should be rather simple, but I keep running into problems. I'm trying to make dependent drop down lists. For example, my first drop down list will contain various product lines, the second drop down list will be dependent on the product line chosen in the first drop down list and contain various sub groups describing in further detail the product line chosen. The third drop down list will contain more sub groups describing in even further detail the product line chosen, it will also be dependent on the product line chosen in the first product line. All of the drop down lists describing the product line will be dependent on the product line. Any information on this would be very helpful. I've been doing a lot of research on my own about this but have come up with no solution that fits this problem. I've attached the file, sheet 3 is probably what you would be interested in. Sheet 1 contains the process I'm currently using, but this is going to be used by more people than just myself so I want dependent drop down lists so that the drop down lists aren't so long. Oh and the color is just showing some of what falls under certain categories such as rotating eletrical, driveline, engines, engine components, etc...
Thanks again!