We have many times wondered how big of an air defense 3 lightning can kill. Here is our experiences summarized:

lvl3 lightning: no experience but i think you could look up the stats and extrapolate from below

lvl5 lightning: any air d, significant slack with placement on lvl5 and below some slack on lvl6 air defense

notes: i missed on a lvl7 air d once because the king wandered over it, distributing some of the damage to the king. Be careful heroes arent on the air d when dropping lightning.

Lvl4 lightning: can at least kill lvl7 air d, our #25 just did this and then 1 starred their #3. Significant risk of missing on lvl6 or higher air d. Best to zoom in if trying to kill a high level air d.

adjacent air d: we have never killed 2 adjacent air d with 3 lightning placed in the middle, it always fails, save once when 3 lvl5 lightning killed 2 <lvl4 air d. The first player thst tried failed, so a lucky placement.

hope that helps.