Multi-Buy Discount

Oh how I’m hoping someone can help!

So I’m using Yith for the first time and already run into some issues!

So I work for a garden centre and we offer a multibuy which is if you buy 3 composts which are individually priced at ￡4.99 you would get them for ￡12 saving ￡2.97 however trying to apply this on Yith has been a nightmare!

If I try and apply the principal that if you buy 2 at 4.99 you would get the third for ￡2.02 which is fine if you buy in multiples of 3 however if you were looking to get 4 that would usually be ￡12 + ￡4.99 = ￡16.99 however it’s coming out at ￡17!

Any help would be greatly received !