First things first, you need to know that three week old baby cannot eat solid food items. There are two things that you can feed a three week old baby, and these are breast milk and formula. The baby cannot have anything other than these food items, and you can feed your baby either breast milk or formula three times a day at a minimum. Breast milk is very nutritious, and has all the necessary minerals and vitamins that your baby needs for growth. It's not a bad idea to feed your baby more than thrice a day, as it depends on each baby's appetite.