Okay, The Dual-Link DVI cable goes from your video card to your TH2go.
Here's the pin out for it:
Using DVI to VGA adapters from the TH2go to the monitors should be fine as long as it's detecting you have all three monitors hooked up.
Also a rule of thumb, you should update your TH2go to the latest firmware and use the latest GXM Software.
EDIT: I might have been mistaken, if 3840x1024 works, so should 4320x900 (uses less bandwidth).
The thing to try would be making a custom resolution with that thread above. You also may want to test your VGA adapters.
If you can, use dvi connections through and through all your monitors.
Stock, no one has been able to get a bigger resolution than 3840x1024 on there TH2Go using an ATI card 5XXX series or newer.
But the custom resolution should do the trick. Hopefully anyway.