I like to have four competetive decks and I am quite used to getting several core sets from FFG LCGs. What benefits would that bring in Ashes. So far I can only think of addition copies of Molten Gold and more dice. What cards do you consider must includes with 3 copies?
personally I don't even feel molten gold is a must include 3 copies. it is a really good card but there are plenty of very good decks you can make without it. also as more expansions come out the need for extra core set copies will be less and less
if you want 4 decks, you might want a second copy for extra cards just in case, and if you will be playing these decks against each other then you will want the second set of dice for sure.
I really cant see a reason to get more than 2 copies.