You haven't read my posts then. I had 3 units at one time and none of them ran together. After cleaning up the paste performance improved but I still had problems. I've since sold 1 unit, but since replacing the USB cable I successfully run 2 U3's and 1 Rockminer R-Box together. Every 4 days or so I do have to cycle power on the U3's and unplug/replug the USBs, but that is acceptable to me.
Just edited my post as I read through this thread again, searching for answers. If I understand you, you had 3 and they wouldn't run together, you cleaned up thermal paste, and then they ran together? You say "performance improved", but what does that mean exactly? If the only change you made to get them to run together was cleaning thermal paste I'll give it a shot.
What happens every 4 days that you need to cycle them? Performance drops? Errors? Zombie?