There is a clear explanation of this in the tips and notes for the unit.
The reason is that 'punt' the money 'pound' is a feminine noun. Welsh numbers used to have feminine and masculine forms for all numbers, these days only the feminine forms or 2,3 and 4 remain.
These are 2=dwy, 3=tair, 4=pedair
Thus 'three pounds' = 'tair punt'