Originally to use SLI the video cards had to be identical – same GPU, same manufacturer and even same BIOS version. From GeForce driver release 80 on nVidia relaxed a little bit and now the video cards can be from different manufacturers, but they must be based on the same GPU. This rule is valid for all versions of SLI (SLI, Quad SLI and Three-way SLI).
All video cards from GeForce 6600 on support SLI, but three-way SLI is only supported by GeForce 8800 GTX, GeForce 8800 Ultra, GeForce GTX 260 and GeForce GTX 280. This mode uses a new bridge as the cards have two SLI connectors.