Depends on what size you're talking about. For example, if you're thinking 3x 21.5 inch HD monitors vs. a 27 inch HD monitor, you'd have 3 times the pixels with a 3-monitor setup. Also, it depends on your graphics card. If you're going to try playing games on a 3-monitor setup in 1080p, you're going to need a pretty good card or two; say crossfire 6950 or 6970 or just one 6990.
I personally am probably going with one 27" monitor so I get a better framerate. I guess it's really personal preference. Hope I helped.
If you play FPS, then go with a triple monitor setup, if you play rpg, then go with a larger, single monitor. WoW heads like singles, as i am one, and FPS love the 3 screen setup for increased vision in the game.
[Edit] To answer your initial queries, it depends on the game. Some games, such as BFBC2, TF2, JC2, Crysis, BF3, F1-2010/2011... enable you to see 'more' of the game world, just like extra widescreen. And others such as Mirrors Edge (Hack), Assassins Creed 2.... simply enlarge the image, not so that it appears stretched but I guess it is effectively stretched across all three screens such that you loose some at the top and bottom. So unfortunately there is no direct answer it simply depends on the game. The AMD website has a list of validated eyefinity games, although don't be bounded by this list, there are many more that work perfectly fine, the best resource is probably widescreengamingforum.com.