Please check /usr/share/doc/NVIDIA_GLX-1.0/README.txt for "BaseMosaic". There's also a configuration example for a 2x2 setup:
nvidia-xconfig --base-mosaic --metamodes="GPU-0.DFP-0: 1920x1024+0+0,
GPU-0.DFP-1: 1920x1024+1920+0, GPU-1.DFP-0: 1920x1024+0+1024, GPU-1.DFP-1:
However, I have no idea how well that works with three displays of different geometries.
Once you got it working, I would appreciate it if you could post some details.