How much puppy food should be given to a 3 month old German Shepherd each day?

By this sort of age you should be feeding him 2 to 3 cups of genuine good brand complete diet or a mix of both complete and meat.
A dog of this age shouldn't be fussy so if he seems too podgy you should cut it down a bit or give it in smaller and more frequent proportions.
If he is fussy take the food away when he leaves it and don't give it or leave it around for him.
A fussy dog is not a good thing and they'll understand in time.