I'm trying to do this as well. When you define a field as a list only one option can be selected.
When I set up 3 different lists and include those as options in the form, it is not handled as mutually exclusive.
I don't understand your answer above to use tags. So if you could explain that more, I'd appreciate it.
And how can I set up the lists so that they behave mutually exclusively (i.e. only one can be selected from the list.)?