How many grains of gun powder are in a 3 inch twelve gauge shell?

With ammunition, the term GRAIN is a measure of weight- there are 7000 grains to a US pound. It does not refer to the number of individual flakes of powder (grains). Load data will vary depending on the weight of shot to be fired, and how hard you want that to shoot- and it also varies depending on which of several powders you use. For example, a 1 oz shot load might use from 20-25 grains of powder X, 22-27 grains of powder Y- and a 1 1/8th oz shot load might use 18-14 grains of X, etc.