As far as I know the only 3 GPU configuration is 2 GPUs in SLI and the other GPu dedicated to Physics processing. Not sure if it exists yet or not but I did hear about NVIDIA trying it.
I know there's Quad-SLI, then I heard NVIDIA trying different GPU combinations, the only problem is gaming applications can't utilize all of the GPUs. So it's potential that can't really be tapped into quite yet. I know one of the far-fetched ideas was to have 2 GPUs dedicated to image processing (games) and then have one dedicated to the physics, and then the other to be in tandem with the CPU and help process.
Not to sure if this addresses your question or not, but it's everything I know about in regards to multi-GPU configurations.
EDIT: Also, the only company working towards the multi GPU configuration with the physics processing and CPU utilization is AMD. Since they bought out ATI it seems like they're going for a completely integrated system. This does help explain the Torenza technology that AMD wants. They want a CPU and GPU to be able to process in tandem with each other, as well as have their DX10 cards to have built in physics processing, and then to have multi-GPU systems for CrossFire platforms.