Treating bound variants as one product at checkout

[PathoStudio]

Hi all

I have a product variation app that treats product variants as separate products (which are then hidden at checkout)

I also run a sale of 20% off when you add 3+ items to cart.

The problem is that these hidden product variants need to be excluded from the 3+ count, but then still need 20% deducted from them once the count is reached.

At the moment I have excluded variant products from the sale, but the 20% discount is not apply to the whole value of the cart. In the attached image you can see that 20% isn't being deducted from the £22 price of the variant. The price in read is what should show for the total.

Any help would be appreciated tremendously!