From Diablo Wiki
Murlocs are a monster found in World of Warcraft. They are aquatic and frog-like, and combine humorous with menacing in a way that's made them one of the more famous inventions of the game. Murlocs have become known outside of the game, for their amusing gurgling dialogue and sound effects, and just generally comic relief nature.
Murlocs in Diablo III?
No. That's what Jay Wilson said when this question was asked, jokingly, during an interview at Blizzcon 2010. 
- Question: Are there going to be Murlocs in Diablo III?
- Jay Wilson: Murlocs? No.
- Question: Thank you! *relieved. much laughter ensues*
- WoW Dev: Aren’t Murlocs the secret class?
- Jay Wilson: Dude! *acting shocked as though a secret has been revealed*
The odds are good that there will be some sort of secret level in Diablo III, but apparently it won't be WoW-themed.
Murlocs were given out as virtual pets at several Blizzcon shows:
- Blizzcon 2008 offered a polar bear mount with a Murloc riding shotgun. 
- Blizzcon 2010 offered a Deathwing-styled Murloc pet named, "Deathy." 
Concept art by Glen Rane.