multiple validation dependent lists based one one source but not on each other?

i am trying to make a set of validation dependent lists where the two lists after the first are dependent on the first, but not on each other. for example in the first list you choose an energy source, like electricity. in the second list you choose a unit for electricity. in the third list you choose the source of the electricity. however if you choose natural gas in the first list, then a different list of units is available in list two and a different list of sources available in list three. i have the fist two lists down but am having trouble adding the third. can anyone help me?