Buy 3 items from a collection and pay a fixed price

[MrJackson2000]

Hi
I am trying to create a automatic 'buy any 3 items for £25' promotion.
All the promotion items will be placed in one collection and the prices of each product will vary

For example a customer will place:
Vase £8
Toast rack £18
Coaster £10
into the cart

At the checkout out I want the final total to be £25.

Like this;
https://www.iwantoneofthose.com/offers/3-for-20.list

thank you

[JanainaFernande]

Did you find out how to sort it out your issue?

I am trying to do exactly the same, no success so far!

Many thanks!!