Can you control frequency
Look under USB Management in cgminer. You can blacklist certain devices. I haven't tried in South so I don't have specific steps for you
not looking to blacklist it (think it disables it when you do), even though I am not sure if it would keep that one at 100 while having the others go to 200+
You can always try running two instances of cgminer, with different parameters.
For example, start the first one @200, with autodetect on, plug in "other two" sticks, disable autodetect after sticks are detected.
Start the second instance @150, with autodetect on, plug in the "middle" stick, and that's it.
Might it be possible to run two instances and use the --usb switch to target specific devices? I haven't used the switch since initially playing around with cgminer, so I might be misrecalling the exact purpose of of it...