The 680 can do this without SLI. Games that support borderless windowed or windowed mode will get you exactly what you want with being able to perform other tasks on the two other screens while gaming on the third.
Some games still take over the whole darn setup, but there are fewer and fewer of these out there as people look to have the flexibility you're looking for. This is independent of what type of video card you choose.
Setting up Nvidia surround is easy. They have a "wizard" on the geforce website that shows you possible monitor configurations:http://www.geforce.com/hardware/technology/3dvision-surround/system-requirements