My Method to use Antminer U3 with BFgminer:

Look in Device Manager* (COM&LPT) which COM-Port CP210X has. Than use this to find the U3:

-S antminer:\\.\COM7

Here is it 7, but use the Number, on which your CP210X (U3) is. Sometimes you have to (HOT-) UNPLUG und PLUGIN USB, but it works fine.

You can add other U3 commands too, but this is used by me to find the U3 directly WITHOUT GENERAL SCAN over all ports.

You have to UNZADIG your PC, if you used ZADIG. Dont use ZADIG, only install CP210X.

* Your U3 should be plugged in (USB & Power) und running.

Addition:
With Bfgminer I have with the U3 nearly Zero HWs (often max. 1/h) and Rejected Shares (max. 0.X%).
With Cgminer I had much more HWs and rejected Shares.