Mike,
I have also found the K3 NB to be ineffective on strong electric fence pulses, using normal settings.
The only fix which I have found that works for CW (apart from changing rig) is the technique that Wayne N6KR described to use a wide roofing filter in the first IF and narrow DSP setting. (see NB messages history)..
This is done by defining  a 6 or 13kHz filter as a narrow filter. Downside is the DSP is then wide open in pileups to the bandwidth of the wide IF filter, however it does silence the clicks!
John G3PQA