It would be interesting to find out what the differences are between the bitmain-flavour cgminer driver for the U3 and the official cgminer driver.

I've never had any luck using the 'official, bitmain' version of cgminer but am getting a good hashrate (most of the time) with the Official non-bitmain CGMiner.

*However* My U3 just dropped out after about 6 hours last night despite being powered through the (750Watt) ATX supply..  had to remove both Power and USB before it would play again.

Waiting for that to happen again to try and diagnose further.