Beast Mastery / Titanstrike Hidden AppearanceThe Hidden Artifact Weapon Appearance for the Titanstrike is called Titan's Reach, and can be unlocked by using the item called Design of the Grand Architect.This turns the gun into a bow, and comes with four colors like the other appearances.
To get the Design of the Grand Architect, you simply need to purchase it from Hobart Grapplehammer in Dalaran. He sells it for 8,000 gold, and there is no other prerequisite to get it. Hobart is found in the engineering building in the Magus Commerce Exchange district.
Marksmanship / Thas'dorah, Legacy of the Windrunners Hidden AppearanceThe Hidden Artifact Weapon Appearance for Thas'dorah, Legacy of the Windrunners is called Ravenguard, and can be unlocked by using the item called Syriel Crescentfall's Note: Ravenguard.
This item can br purchased from Veridis Fallon in Azsuna for 1,700g once you are Revered or higher with the Court of Farondis. Here's a guide to getting reputation with the Court of Farondis, if you are still not revered.
Survival / Talonclaw, Spear of the Wild Gods Hidden AppearanceThe Hidden Artifact Weapon Appearance for the Talonclaw, Spear of the Wild Gods is called Bear's Fortitude, and can be unlocked by using the item called Last Breath of the Forest.
This item is a drop from Ursoc in The Emerald Nightmare raid. He can drop it on any difficulty, including LFR, but the drop rate is pretty low, so you will probably need to run it a few times before you are able to get it. You can loot the Last Breath of the Forest regardless of which specc you are playing as, so it's not limited to only survival hunters.