3rd class quest
The 3rd class comes from the sagas — Saga of … quests from level 76. They work the same for every class: six tablets, the Ice Crystal, the Divine Stone of Wisdom and a fight with the Archon of Halisha.
Humans
Duelistafter Gladiator 
Dreadnoughtafter Warlord 
Phoenix Knightafter Paladin 
Hell Knightafter Dark Avenger 
Adventurerafter Treasure Hunter 
Sagittariusafter Hawkeye 
Archmageafter Sorcerer 
Soultakerafter Necromancer 
Arcana Lordafter Warlock 
Cardinalafter Bishop 
Hierophantafter Prophet Class guides:GladiatorWarlordTreasure HunterSorcererNecromancerBishop
Elves
Eva's Templarafter Temple Knight 
Sword Museafter Sword Singer 
Wind Riderafter Plains Walker 
Moonlight Sentinelafter Silver Ranger 
Mystic Museafter Spellsinger 
Elemental Masterafter Elemental Summoner 
Eva's Saintafter Elven Elder Class guides:Sword SingerPlains Walker
Dark Elves
Shillien Templarafter Shillien Knight 
Spectral Dancerafter Bladedancer 
Ghost Hunterafter Abyss Walker 
Ghost Sentinelafter Phantom Ranger 
Storm Screamerafter Spellhowler 
Spectral Masterafter Phantom Summoner 
Shillien Saintafter Shillien Elder Class guides:Shillien KnightBladedancerAbyss WalkerSpellhowlerShillien Elder
Orcs
Dwarves
Fortune Seekerafter Bounty Hunter 
Maestroafter Warsmith Class guides:Bounty Hunter
How the sagas work
The 3rd class quest is a saga: every class has its own (Saga of the Spectral Dancer, Saga of the Doomcryer and others), but the steps are almost the same for all. You take it at level 76 from your class's NPC, then go through six Tablets of Vision, six Resonance Amulets, a fight with the Archon of Halisha and Death Lord Hallate. You change class at the end of the saga with the same NPC.
You will need two items from other quests in advance: the Ice Crystal (The Finest Ingredients — Part 1, level 73) and the Divine Stone of Wisdom (Magical Power of Water or Magical Power of Fire — Part 1, level 74, only with a level 2 alliance with Ketra or Varka). The longest part is 700 Halisha's Marks at the Shrine of Loyalty; how many monsters that takes depends on the quest rate.
The saga rewards 5,000,000 adena, the Secret Book of Giants and experience.
FAQ
What level is the 3rd class quest?
Level 76 — the class change at the end of the saga needs the same level.
What do I need in advance for the 3rd class?
The Ice Crystal and the Divine Stone of Wisdom from other quests, and a level 2 alliance with Ketra or Varka for the Divine Stone.
How many monsters do I have to kill?
Mainly 700 Halisha's Marks at the Shrine of Loyalty: 700 kills at quest rate x1, 70 at x10. Instead, a party can kill the Archon in the Four Sepulchers.
Can I do the 3rd class on a subclass?
Yes, the saga of your class can be taken on a subclass too.
Can I get the 3rd class without the saga?
Yes, if the server has a Class Master: it changes your class for adena.