Under normal circumstances, a mother dog takes care of her puppies until they're 8 weeks old or more, weaning around 4 weeks of age. When a puppy is orphaned around week 3, you'll have to step in. You'll have a lot of work to do for a few weeks, but your task is much easier with a 3-week-old than with an orphaned newborn.
Keeping Him Warm
It's important to keep your puppy warm, especially if mother isn't around to provide body heat. Keep him in a room with a temperature of approximately 80 degrees Fahrenheit. Within a week, lower the temperature to 75 degrees. Placing a hot water bottle under the towels or blankets in the dog's bedding will do if you can't provide a climate-controlled space, or you can use a heating pad. Just keep the latter on a fairly low setting so it can't burn the puppy, and make sure it isn't a fire hazard.
While you might have to bottle-feed your puppy for a short time, three weeks is about the time a puppy can start eating solid food. If your puppy is a small breed, he might require bottle feeding until he's a little older. Purchase commercial puppy milk replacer from your vet or a pet store, preparing it according to the directions on the container. You'll also need nursing bottles, preferably those specifically designed for puppies. At 3 weeks, puppies require bottle feeding approximately every six hours. You can try getting the puppy to drink milk replacer out of a bowl at this point.
Your vet can recommend a high quality puppy food for your little pal. Initially, you might want to mix puppy food and milk replacer so it forms a gruel. Generally, you'll feed the puppy about four times daily, but your vet will provide specific feeding advice.
Weigh your puppy every day, recording the amount. At 3 weeks, a puppy requires between 80 and 90 calories per each pound of body weight, according to the American Kennel Club. The following week, the puppy needs more than 90 calories per pound. After the fourth week, he won't need commercial milk replacer and his entire diet can consist of solid food.
Three weeks of age is just about the time puppies start urinating and defecating on their own, without needing the stimulation of the mother dog's tongue to get them going. Watch him to see if he "goes"on his own after eating. If he doesn't go within 15 minutes or so of consuming a meal, dampen a soft washcloth or gauze piece in warm water and rub his anal area and lower abdomen in imitation of the mother's licking. While a puppy this age is ready to eliminate on his own, it's too early to start any housebreaking. You'll just have to clean up after him for a few more weeks.
At about the age of 3 weeks, your puppy is ready for his first deworming and possibly his initial vaccinations. Most puppies don't start receiving vaccinations until the age of 6 weeks, but if your vet suspects your puppy didn't receive sufficient maternal antibodies in his mother's colostrum, the vet might begin the regimen earlier. This checkup is an opportunity for your vet to examine your puppy and ensure he's doing well, even without a canine mom.