Brewmaster / Fu Zan, the Wanderer's Companion Hidden AppearanceThe Hidden Appearance for Fu Zhan is the Ancient Brewkeeper, which is unlocked from the item Legend of the Monkey King.
This item is randomly dropped from the Bubbling Keg. In order to get this you need to pick the Brewhouse Talent at your Class Order Hall, which lets you tap a key every day. So basically you have a small chance to get the Legend of the Monkey King each time you use the keg, so just keep checking this keg every day until you get the item.
Mistweaver / Sheilun, Staff of the Mists Hidden AppearanceThe Hidden Appearance of Sheilun is called the Breath of the Undying Serpent, and you need an item with the same name to unlock it.
The Breath of the Undying Serpent drops from any Dragon of Nightmare in The Emerald Nightmare raid. It can drop from either Ysondre, Lethon, Taerar or Emeriss on any difficulty, including LFR. The drop rate is pretty low, so expect to farm this raid for a few weeks before you get it.
Windwalker / Fist of the Heavens Hidden AppearanceFor the First of the Heavens, the hidden appearance is called Stormfist. You unlock it by using an item called The Stormfist, which is a random drop from the endchest in the Whithered Army Training scenario / World Quest. You need to have progressed pretty far into the Suramar questline to be able to do the scenario, so make sure you are working on this chain.
It is not certain if you get a higher drop chance by getting a high score on the scenario, but some people think you do. So if you are going to do this, gather up lots of Ancient Mana (here is a nice guide for farming Ancient Mana very fast) and go to the scenario. If you don't get it, bad luck, just keep doing it until you eventually loot it from the chest at the end of the scenario.