Bashiok (Blizzard CSR) responded to a post on the official Diablo 3 forums asking if the game would support a 3-monitor setup:
It's a single-monitor only game, and that's because the game literally only pulls info from the servers for a specific distance away from the character for bandwidth reasons. The game is also built with those limitations in-mind: edges of maps don't extend way out.
Also, being able to see way out to the left or right of your character really wouldn't help that much. Triple monitor is cool for first/third person where you're getting a larger peripheral view, but in a fixed isometric camera it doesn't make a whole lot of sense.
Here's the link:
http://forums.battle.net/thread.html?topicId=27822632495&sid=3000
Pretty dissapointing. That, along with the real money auction house, online only play, and other dissapointing announcements pretty much seal the deal for me :(