I just want to say before I make my point that I have zero programing knowledge so I might not understand everything on that aspect but... horisontal span or not at the end of the day even if the only solution is to use SoftTH or TripleHead2Go, games like rFactor do this already with no issues on Vista with SoftTH.
Basically all we need to do to add proper triple screen support is this:
In a config file or the game itself there is an option for the number of monitors, 1, 2, 3 or 4.
All this option does is control the # of camera's on the drivers helmet.
The screen would then be equally divided into 2, 3 or 4 depending on how many monitors you have picked.. you can then fine tune the angles in the same manor you would fine tune the rear view mirrors in a real car. press a button to move the left camera, use your keys to move it up, down, left, right, tilt and angle. Once your left screen is aligned with center, you press the button to do the right camera, align it as well and your off to the races.
If you select 4 monitors the fourth would be used for a rear view mirror. You should be able to do two things with this view. Use it like an actual rear view mirror on a small widescreen LCD and mount it above your center screen OR if you have the room and are running projectors you can mount it behind you and use 3 REAL rear view mirrors to show whats actually being projected behind you.
In order for that to work you need to be able to reverse the picture so you can use it as a rear monitor or rear view mirror monitor.