Herobrine is a myth from the pc version of minecraft. It was later turned into a mod, but has never officially been in the game. Herobrine is a NPC Ghost character that looks exactly like Steve but has all white eyes. He teleports around and builds simple structures.
The mention in the 360 version is an obvious continuation of the joke. Notch has never confirmed Herobrines inclusion in any version of the game. --- Still not buying Diablo 3!
check the wiki. Remember the triforce in ocarina of time?
I know who Herobrine is and I don't get the connection to the triforce at all... --- I don't have a signature but Instead I write this so that no one accidentally doesn't read the last line of my posts.