Never had an issue with any countershaft nut before. Here is what I would say though, if you have done it to three in a row there is something wrong. Either your over torqueing them, or your not installing the sprocket and nut correctly, or the threads on the shaft are damaged. But you should not try it again until you figure out what your doing wrong.
By saying you checked it, meaning your torque wrench, did you have it checked or how did you check it. Because it takes special calibration tools to check that a torque wrench is working correctly.