OK , have you tried (if you can) setting all 3 monitors to the same resolution ? because if you can and it works , it would be a step forward .
how about your options.lua ?
the graphics portion of it should look something like this (adjust for your chosen level of details) but this should be your aspect and resolution ;
try first scaling down to smaller resolution of 1280x1024 on the 3 monitors (if you can )
here assuming it would work with 2 different resolutions 1280x1024 1920x1200 1280x1024;
another option , if your center one can't come down would be to put all monitors at 1920x1200 ,if the side monitors can do it fine but if not use 1280x1024 in softh.cfg (you can even scale down more for added performance) here ;
; 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
here's the options.lua for that 3rd option , all at 1920x1200 ;
there is also a softh.cfg setting for adjusting manually for different size side monitors if "auto" fails ; yes it can be a negative value in pixels .
; sideExtraWidth: Width difference between middle and side monitors, or 'auto'
hope you can have results with all on 1280x1024 then at least you could have a starting point .
hope that helps