Nvidia surround still doesn't support mixed resolutions, so there is no ideal solution for this kind of setup. As scavvenjahh mentioned you can use
SoftTH, but note that this doesn't work for DirectX 10 and later (OpenGL and Mantle are also unsupported).
Another workaround is to make a custom resolution for the visible portion of the central display and then use a borderless window. See
imusrt's blog (particularly the Manual PLP Instructions) for how to accomplish this for specific games. UI position etc. may vary from what is described (your reduced setup is wider than PLP) but the steps are about the same. These methods won't work for every game.