The instructions for your Seltzer Kit are correct in calling for 3 carbonation drops per 750ml bottle as seltzer is served at a higher carbonation than beer.
For most beer styles, 2 carbonation drops are all that are required per 750ml bottle as they are typically served at 2.5 volumes of CO₂. As well, most English and higher alcohol beers are served between 1-2 volumes of CO₂, which is closer to 1 carbonation drop per 750ml.
Where Wheat, Fruit Beer, and Seltzers differ, are that they are typically served at 3-4.5 volumes of CO₂, which is equivalent to 3 carbonation drops per 750ml bottle.