Jeff Atwood recently posted about using three monitors. He's a big fan of multiple monitors.
Personally, I'm not so sure. A few years ago we upgraded almost everyone at our company to dual monitors. I was one of the few hold outs.
At that point, one of my arguments was that I preferred one large monitor to two smaller ones. Even if the surface area is larger, two monitors isn't so good if you want one large window (e.g. Eclipse or Lightroom). Of course, if money isn't an issue, you could just have two (or three) large monitors.
To be clear, I'm not saying there aren't good, valid uses for multiple monitors. Studies have shown they can increase productivity. But I'm not sure that translates into normal usage. As the saying goes, "In theory, theory and practice are the same. In practice, they are not."
I think the key issue is how people use multiple monitors. I suspect that many people use them in ways that are counter productive, that just add distraction and interruptions to their environment. I think you should be closing your email and chat and Twitter and Facebook - not using multiple monitors to keep them constantly in your face.
Of course, it's virtually impossible to wean people away from keeping email etc. open all the time. It doesn't matter how much you talk about the cost of distractions and interruptions. Regardless of the excuses they give, I think the problem is that a lot of people welcome these distractions. After all, it's hard work to focus on one task for extended periods.
If you're going to focus on one thing at a time, then do you still need multiple monitors? Occasionally, I would like to have documentation or the program I'm working on open as well as my IDE. But I find virtual desktops to work pretty well for this.
I do agree that ending up with one monitor straight ahead is an advantage of three monitors over two.