What does the Matrox GMX Control Panel list as available resolutions?
What does the OS system preferences panel say for the output resolution connected to the TH2G?
Does it say 3840x720 or something else?
It is possible that your graphics card may not support 3x 1280x720 resolution...
If it does - and all above is correct, then:
In catalyst:
for 3 x 1280x720 you will have to set each mix window manually to 1280x720 then manually position them on screen next to each other:
eg:
mix 1: pos 0,0
mix 2: pos 1280,0
mix 3: pos 2560,0
see picture attached
The list is a quick 'setup' for common mix window sizes - but not a 'definitive' list - other resolutions are available manually
Call me if you need some assistance
Simon