no luck... it's probably just config stuff... I have been trying to run COD4 on the 3 monitors. what I ended up with was the right res for the 3 monitors, but the game being really squished horizontally on the centre one. that and not being able to move the mouse past the centre of the screen. let me know if you have any quick fixes, otherwise I'll start a new thread in the right place...
What's probably happening is that you're running the resolution of the two 19" monitors onto the 21" one, and it's therefore having to stretch a non-native resolution to fit.
Try seeing if your 19" monitors can downscale the resolution from the 21" one (look in the config settings of SoftTH, although the monitors probably won't be able to).
Otherwise, SoftTH should be able to run the side monitors at different resolutions (although that will require some special configurations of the .ini file).