Does this mean you need to include Pillar/Pendulum of all 3 elements?
no, "cards" mean any card
Also, do 0 cost cards (e.g. Ray of Light) count as 1 of your elements (even though you would need Another card of that element)?
Decks must have at least 3 non-zero quanta cost cards from each element used.
As far as you don't go over the limit stated above, it'll be ok
I'm not a TO or anything, that's just what the rules say