Hi - Thanks for asking ahead!
The Libretto W105 is an extremely interesting little PC with no keyboard, instead having two touchscreens on the inside.
Unfortunately, there are two potential red flags with this system:
The 1.2Ghz processor is below our recommended minimum for running USB graphics on Win 7. Rendering will be sluggish at higher resolutions.
The dual touchscreen requires custom touch drivers, and often those drivers will freak out when a third display is around (at the very least, it is problematic to move the pointer to the third display)
Other than those two problems, however, actually the USB graphics device will not be limited to any particular resolution - it can hit higher modes, completely independent of the main GPU’s limitations. But, again, the problem will be that rendering will be slow at those high modes, mostly because of the CPU.
So while your original concern isn’t a problem, I think these other two issues are important, and unfortunately wouldn’t recommend a USB graphics solution for the machine for this reason. I know that might leave you with no solution at all for getting to a 3rd monitor, but I’d rather save you the frustration of a solution that might get tantalizingly close, but have problems.
Sorry, but hope that background helps. Thanks again for asking ahead!