Can I set up a fixed value (fixed price) discount?

Yes, BYOB can support a fixed value or fixed price discount. 

By default, Shopify only supports fixed-amount off, percentage or buy x get y for product discounts. There is no way to set up such a discount in Shopify.

BYOB uses variants to adjust to a flexible bundle price. As a result, you can set up discounts like any 3 for $X in a collection. You can select the discount type of Price in the app. 

BYOB is one of the very few Shopify apps that can do so.