added install commands for proprietary nvidia drivers for 3 card scenarios.

For GeForce 400 series cards and newer [NVCx and newer], install the nvidia or nvidia-lts package, available in the official repositories.
For GeForce 8000/9000 and 100-300 series cards [NV5x, NV8x, NV9x and NVAx] from around 2006-2010, install the nvidia-340xx or nvidia-340xx-lts package, available in the official repositories.
For GeForce 6000/7000 series cards [NV4x and NV6x] from around 2004-2006, install the nvidia-304xx or nvidia-304xx-lts package, available in the official repositories.