I have been using the SofTh for a few months now and this is my confg:
[config]
; deviceIDLeft/right: Direct3D device ID's of left and right monitor, or 'auto' for automatic
; secondaryWidth/Height: Left&right monitor resolution, or 'auto' to use same as middle screen resolution
; secondaryRefresh: Refresh rate of left & right monitors, or 'auto' for automatic
; secondaryFormat: Secondary device image format, 'RGB32', 'RGB16' or 'RGB16D'.
; borderSize: Size in pixels of frame border between monitors that is discarded
; noHotkeys: Set to 1 to disable all hotkeys
; noOSD: Set to 1 to disable on-screen display when changing settings (Also disables GDI+ initialization)
deviceIDLeft=auto
deviceIDright=auto
secondaryWidth=auto
secondaryHeight=auto
secondaryRefresh=auto
secondaryFormat=RGB16
borderSize=128
sideExtraWidth=-336
noHotkeys=0
noOSD=0
Then just add your size to the bottom under modes to get it tripled:
[modes]
800x600x60
800x600x75
800x600x85
1024x768x60
1024x768x75
1024x768x85
1280x960x60
1280x960x75
1280x960x85
1280x1024x60
1280x1024x75
1280x1024x85
1600x1200x60
1600x1200x75
1600x1200x85
1280x800x60
1280x800x75
1280x800x85
(add yours here) 1920x1080x60
(add yours here) 1920x1080x75
(add yours here) 1920x1080x85
auto