Reveal three. It's bogus since some dark arts cards do insane damage AND you draw another dark arts? Did they even play test/balance this game? We have lost every year once and sometimes twice.
This is the reason we have lost year 5 repeatedly, 3+ times I think.
You can end up drawing 3-5 dark arts cards in a single turn, all of them some of the worst you can draw, and typically players start ending up with hands of 1-3 cards each.