Why is there soft mutation after dwy but not tair or pedair?
The historical reason: because tair and pedair formerly ended in a sound that didn't cause soft mutation. (I think it's usually final vowels that caused soft mutation, that mostly dropped later but the word kept its mutating effect.)
That makes sense ... I think! Thanks
Tair and Pedair don't cause soft mutations. Dau/Dwy cause soft mutations on all nouns that come after them, but Tair (note only the masculine form) causes aspirate mutations on nouns that follow it.
Can't this sentence be translated "How much is she? Three pounds?"
It would be pretty normal to hear "Three pound" in English, with the plural dropped.