I know this isn't strictly connected with maths (it has more to do with electronics) but I'm hoping someone here could at least shed some light on it.

I've got a problem to solve that involves three containers which should be filled with sand, each of them has a sensor attached at the same level, so if the amount of sand drops below that limit, it will be refilled. If all were empty, the refilling arm would first go to 1, then 2, and finally 3. Something like this:

I'm not sure how many states are needed, I'm guessing at least 3 (probably 4: 1st empty, 2nd empty, 3rd empty, all full). Could anyone help me make a state table or state diagram to get things going? Also, would I be better off using a Moore or Mealy model? Help greatly appreciated.