Hello Richard,
We have created three radio buttons (Daily/monthly/3 monthly) using characteristics which changes price in the description depending on which one is selected. This is fine and what we need.
My bad, I didn't saw that you where using characteristics for your 3 radio buttons, there is no option to restrict custom item fields regarding variants.
But if you really want to do it like this, the solution will probably be to directly edit the "show_block_custom_item" file of the "product" view of your front-end template via "Hikashop->Display->Views" (some development skills will be required).