The Three Tauren Brothers is available as soon as Baylon Plains is finished.
After entering Baylon Plains again, this time the dungeon needs to be completed and the boss fight needs to be won in order to complete this quest.
Click on [Campaign] -> Select the [Baylon Plains] -> Kill [The Brothers Tauren]. The Brothers Tauren are a group of thugs terrorizing the Baylon Plains. Defeat them at once!
When the end of the dungeon is reached this time, instead of Karlan on his own, all three Tauren Brothers are there. The following dialogue starts:
Karlan: Mwahahaha! Brothers, look! That annoying human is back for more. Idiot!
Dinah: These are the Brothers Tauren, villains taking the most advantage of the turmoil in the world.
Hero: Yeah, I kind of figured that out. But their reign of terror ends now!
Karda: You arrogant fool! Come over here and let us show you what happens to those who mess with the Brothers Tauren!
Once completed, players are rewarded with 8.000 EXP.