This might be a dumb question...Im assuming that you have the cards in a Triple SLI config to consolidate and maximize power and performance. Couldnt you run one monitor off of each card and span it as one large 3x1920x1200 resolution monitor??? Im a stickler for running my monitors at the largest possible resolution during gaming...especially RTS,MMO and FPS. Or couldnt you run two cards in SLI mode, have them render the image and use the third card to get the third monitor and run all at native resolution...
Im new to this surround gaming thing so...
Currently Windows does not support 3D rendering to displays rendered on multiple GPUs. SoftTH is a hack to get around this limitation, but only one GPU does the calculating while the other is for output only.
ATI is developing a system that works like you describe, but there's nothing out there right now that can achieve it.