I am running the same resolutions - 1200 in the middle and 1080 on the sides.
Eyefinity will only run at the lowest common denominator, so you will need to run 5760x1080 when gaming, but you can use the proper resolutions when not gaming with only a couple of key presses to move between them.
Firstly set up the 1200 as your main, and the two 1080's as Extended. This will give you your normal desktop/internet/working setup. Now hold the Windows key and press P. Press P until you get to "Duplicate" and then release Windows. Now go into CCC and set up Eyefinity.
Now, when you want to game, press Windows+P and select "Duplicate", when you want to work/surf, press Windows+P and select Extend.
Running the "wrong" resolution on the 1200p screen isn't perfect, and your middle screen will be stretched by 10% vertically, so stuff there will be thinner and taller than it should be. I know this happens, but after a few seconds in a game you honestly forget about it completely. Nobody else who has played on my PC has ever noticed anything is amiss. I would say definitely go for it and, if it really does bug you after a while, sell the Dell and get another 1080p screen (or sell the two 1080's and buy 2 more Dell's - but that's gonna hurt the wallet!)
