Tom,
To successfully do a span both monitors need to have the same resolution.
With the DualHead2Go appearing (to Windows) as a single 3840X1200 monitor and the other single monitor being a 1920x1200. This setup will not allow spanning to occur. You would still be able to use all three monitors on the desktop in 2D but games will play either on the two monitors connected to the DH2Go or the single monitor direct connected to your video card whichever is primary.
I'm unsure on the scaling of the triplehead signal up to the size of the 26" panels. If you have access to a 26" panel the easiest way to see what it would look like is set 1680X1050 as your resolution on a single monitor and see how the scaling looks.
Your other option is to use SoftTH
http://www.kegetys.net/SoftTH/ which is a software solution to the triple monitor problem. SofthTH is not limited to the 1680x1050 resolution (of the DTH2Go) due to the monitor signals not all being fed through a single Dual-Link DVI port. There is at least one guy I know of on there with three 30" panels. Compatability can be a bit sketchy for certain games but there are a lot of people using it and able to help provide solutions for alot of the current generation games. I used this software before I bought my DTH2Go and found it very useful, if a little buggy, it does take a bit of trial and error and tweaking the settings to get it to run. There is also a slight performance hit due to it being software based.