As title states, with best regard! Disc Priest / Holy Paladin / Resto Druid / Resto Druid or Disc Priest / Holy Paladin / Resto Druid
I´d say go 4 for progress and you can always go down to 3 if required
For progress I would say use 4 until players are more consistent in p2 and p3. Our alt run used 3 healers for 15 players on heroic blackhand this week and it felt pretty comfortable.