“Note that for this story to work, your slots should be unfeaturized. If any of these slots are featurized, your story needs to include slot{} events to show these slots being set. In that case, the easiest way to create valid stories is to use Interactive Learning.”
I assume you add the slot to the required_slots function of your form.
Another option is to use an unfeaturized slot and a validation function to make sure it only has one of those three values.
In any case, I think you want to change the flow of the conversation based on that slot, so read the part in conditional slot logic.