While a user is editing a table row, I am trying to use the slection of two field to control selecti

We’ve built a custom corrective action process where within a table different business unit (field1…cable plant, hardware plant, etc.) would select responsible area (field2…quality, manufacturing, engineering, etc) and then a problem cause (field3).  So we need to have the problem cause dependent upon the responsible area which is your standard SF dependent picklist.  My issue is that I also need the problem cause dependent on the business unit since each (as an example) would not have the same problem cause when the responsible area is quality. I am thinking a snippet might help with setting field 3 after fields 1 and 2 are set but need this to happen as the row is edited. I could have the relationship in the snippet…putting in a custom object querying is even better from a maintenance standpoint.