What are you planning on feeding him? 3 week puppy cannot eat solid food or kibble food. You need to get milk replacement and probably a bottle or tube to feed the puppy. Puppies eat frequently and they grow faster and typically eat 8 to 10% of their body weight distributed throughout the day by a 4 to 6 feedings. Since, it is a 3 week old puppy.
This website gives good information including the amount, quantity and other information that may help you.
http://www.animalsdr.com/artic les/info/feedingorphanpuppies. html