Go to this page and fill out the information as, geforce 670, 2-way sli, 2D surround, landscape or portrait. It'll show you where to plug in the monitors.http://www.geforce.com/hardware/technology/3dvision-surround/system-requirements
Hopefully it's simply a matter of configuration and not some defect.
And just so you are aware, all 3 monitors must be using digital connections. Anything but VGA, d-sub or DVI-A.