Have you flashed the unit with the latest firmware? I mean the latest firmware from Bitmain?
if not flash it first and try it again because 3TH/s is a huge drop from 14th/s you might also need to check and maybe it's overheating that is why you just got the 3TH/s.
Check the kernel logs why the hashrate drops a lot.
I did actually speak to Support at Bitmain and they mentioned that it looks like its something to do with the Fans - they mentioned unplugging and then plugging the fans back in and doing a reset of the device.
So i'll give both a go and see if either work them.