"were born as twins"
Now, see, that's curious. Makes me wonder if, rather than being cursed to only bear two children, they were cursed to have only two pregnancies per generation.
Thus, the "twin" aspect would allow for an additional child without affecting the total allotment of pregnancies, finally allowing them to break the restriction and reach a total of three offspring.
Well, the other possible explanation is just that the big ol' magic circle at the start forced a third child on its own, but that seems a dull basis by comparison.
Comparatively, the "twins" element seems a rather nifty way of justifying the matter.