Group discount

So here is what I want. I have books that are sorted in collection (grouped product) so I want if one use buys the whole collection to play less.

Ex:

The collection 1
– Book 1 – 20$
– Book 2 -20$
– Book 3 – 20$

So if he buys
Book1 + Book2 = 40$ or Book 2 + Book 3 = 40$ or Book1 + Book 3 = 40$

But if he buys Book1+Book2+Book3= 45$ (15$ each)

So how can I achieve this? There is one option, adding the collection as a product for 45$ but one more problem how to suggest to them if they but 3 separate that they can buy the whole collection for less?

Thank in advance

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