When they are ready.
Unlike other Classes, those 3 require art resources to be playable.
Every new Paladin race needs 2 new Chargers.
Every new Shaman race needs 4 new Totems.
Every new Druid race needs a bunch of new Druid forms.
I imagine they will make all the relevant resources for one Class in a bulk and then unlock all combos for that Class, then move on to the next Class.