What should a 3 months old German shepherd eat?

Well I think this varies, but my puppy who was 3 months old ate beef and NO chicken, because it caused him diarrhea. Potatoes are good, turkey also works. You should try the puppy on a grain-free diet, that works best. Many natural ingredient food brands are good, like Nature's Variety. I had my puppy on Nature's Variety. Don't forget to give your puppy a bit of diversity and variety in his or her food, they can't just eat the same flavors for forever. Don't transition from one food to another kind very drastically, though. If you are switching, first put 25% of the new food in with the old, then 50%, then 75%, then 100%. If you switch all at once, it can cause diarrhea.

Hope that helps and good luck!