The ADA substrate bags are measured in liters, which is volume. One liter is 61 cubic inches. With 3 inches of substrate on a 12x12 tank, you'd have 432 cubic inches, and 3L of substrate is 183 cubic inches. So you'd need three 3L bags of Amazonia (or one 9L bag) with no rocks or other substrate to get 3 inches of substrate.
This is good information. Substrate depth needs to be considered. There wasn't any listed in the original post.