Dependent Drop Downs with like values

I have created working dependent menus with 3 drop down menus. The problem I am having is that they are all like values.
 
So menu 1 is A, B, C. But I need menu 2 to be B, C if A is chosen in menu 1, A, C if B is chosen, etc...
 
And menu 3 needs to be whichever choice was not picked in the first 2. Each menu needs to be a different letter. I can get the first 2 to work correctly, but I can't figure out how to get menu 3 to give me the letter that was not picked in menus 1 or 2.
 
I can't use rawValue from menu 2 since i is also dependent on the menu 1 choice.
 
Any help would be appreciated. Thanks