You can do it with one video card and a Matrox TripleHead2Go box.
Look here: http://www.matrox.com/graphics/en/products/
Yeah, but note: 5760 x 1080 runs at 50 Hz refresh rate-http://www.matrox.com/graphics/en/products/gxm/th2go/resolution/resolutions.html
Some monitors cannot support that refresh rate, and most run at 60. The devil is in the details, and may soon be in your rig if you attempt this without checking your monitor specs.