My 3 LSAT Scores are
160
156
166
I know. it’s nuts. what should i write for the addendum for the variance in scores? please help!! 
I honestly do not think you’d need one. If you do, just explain that there was a definitive reason for underperforming on test 2 (sickness, life event etc).
Congrats on the big jump. You just passed about 25 T1 medians. I dont think one is necessary either. Why/when did you score a 156? If its because you hadnt peaked, you shouldnt write one and just be happy.