Three week old puppies cannot be potty trained yet. They are too young.
When the puppy is old enough to start potty training (usually around 10 - 12 weeks), you ALWAYS use positive methods. Punishment based methods can cause some dogs to feel that going potty is bad anytime, anywhere. Using positive methods will make the process go much faster.
If you are in the USA, Petsmart stores carry a great, little book that will teach you how to properly potty train a dog. It's called "Potty Training is Possible."
PLEASE do not attempt to potty train a three week old dog. I don't have room here to go into it all, but a puppy's brain is developing. If you use punishment on a three week old puppy, you will be creating a highly negative impact on that growing brain and the connections being formed. A puppy that old also won't have any motivator (ie reward) available for you to use in positive training either. In addition, their bladders are not capable of holding urine at that age.
Wait to begin potty training until the puppy is old enough to understand rewards, and the bladder is capable of holding urine for several hours.
I strongly suggest that in addition to the "Potty Training is Possible" book that you look into a good general book on how to raise a puppy. Look for a book that promotes positive reinforcement training techniques.