Have Cell Allow (Range) Or Multiple Cells To Be Referenced

Trying to get a problem solved to have a % discount & 'flat' $ discount apply to cell(s) referenced. Product A, B & C are available in 3 different materials. Objective is to allow each material's Max Discount column to apply discount(s) referenced in cell J3 & K3, but only in the order of applying the percentage discount BEFORE the 'flat' $ discount.

Formula in cell C3 works IF the cell "Range to apply discount" I3 = "B3". Works just fine if I allow only one cell to be displayed in I3, but ideally, I'd want this to allow the said discount(s) in cell J3 & K3 to apply to any cells mentioned (for instance, for Product A Material 1, Product B Material 2).

I had tried a drop-down menu using Data Validation, which in my actual project allows me to select ONE cell at a time (out of my list of options), which is okay, but it would be extra useful to have a checkbox option to select which ones to apply the same discount to, instead of creating a massive embedded IF function to have each product in each material find whether it's cell is referenced in 20 different places (if I just copy the already functioning single cell reference tool).

The scale in which I'd like to use this in would be to apply specific discounts for one product, but a different discount for another product or material, and allow the input cell I3 to include a checkbox drop-down option (like the filter/sort), but not remove the data in the table (Range A2:G5 in this example), so I can hide any unnecessary columns and print, showing certain products & materials having a 10% discount, while others having a 20% discount, with an additional $5 off, etc.

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