Product with 3 attributes, but some variations only use 2 — I want to disable the unused attributes

I’ve been searching for a while and haven’t found any answers for this, but it seems like it should be simple…

Here’s the product in question.

There are 3 attributes, as you can see. However, not all variations require all 3. For example, if you pick "Dummy" for the Function attribute, there is no Backset option. But I can’t find a way to disable Backset per variation.

The best I could come up with was adding an option "none" to the variation. But ideally, backset wouldn’t appear if you choose Dummy.

Is this possible?