I would actually recommend that you restrain the game to 1 monitor and use your other 2 screens to quickly look stuff up, in combination with windowed (Fullscreen) mode.
WoW is not really a game that lends well to using 3 monitors. The main reason to have a game spread over 3 monitors is so you have more peripheral vision. This is mainly useful for highly competitive games like multiplayer FPS and racing games, where that peripheral vision can alert you to someone who is flanking you. However, unless you're doing a large amount of PvP, having that kind of vision is not actually needed.
In addition, due to the 3rd person nature of WoW, you already have a lot more peripheral vision than in most FPS games.
Now, suppose that you really want to go through with this, I would make the 2 side screens show a ton of statistical data. I'd specifically look into combat-focused addons like Skada. You can theoretically make a dozen Skada windows, one to show each window, so you can get live statistical data about who is slacking, who is standing in the fire too much,...
You can also use it to have a bunch of extra chat windows. One for guild chat, one for the various group chats, one for zone and trade chat, one for addon messages,...
Finally, most windows displayed by addons and the game itself also can be moved there, allowing you to keep your main window clean.
However, do not use these side screens to display critical data. You don't want your only raid frames to be on there, nor your boss timers, nor your action bars. keep everything you need for combat on one screen.
One word of advice: having such a large screen with so many addons will likely have a noticeable effect on your framerate, and not a good one either. You will also have to make your UI from scratch, since most premade UIs, like LUI or ElvUI, will not cope well with your special setup.