I guess you treat the unit as one display but with 3 times the horizontal resoltuion of one of the monitors and it does the splitting for you, So if each monitor was 1024x768*, you would drive the unit as if it was a monitor of 3072x768 - I guess the iMac could probably do this with sufficent video RAM (allow 4 bytes per pixel R, G, B and alpha), but you might need to to use a special utility for custom resolutions such as switchres.
* I know this is a bit low nowadays, but just an easy example