You could potentially control this with a macro where you set an initial selection based on the change in listbox C and toggle the "Always one selected value" checkbox for Listbox A/B depending on the selection in C.
However, this is pretty cumbersome and not very intuitive for the people using the application.
I would recommend leaving the selections free and then use calculation conditions on the visualizations that you want to do based on the selections so that if you've selected more than one value in both A and B the chart will not calculate and display a helpful message for the required selection base.