At 3 weeks old, your baby should be drinking around 2 ounces of milk per feeding every 3 or 4 hours, though your baby's milk intake might vary according to factors like their weight and whether they're drinking breast milk or formula. In terms of milk production, you should aim to pump between 25 and 35 ounces of breast milk within a 24-hour span if you're exclusively pumping.
However, it's worth noting that this isn't possible for every woman, and that not all parents have access to breast milk, so don't worry too much about what you 'should' be doing. All that matters is that your baby is happy and healthy, whether that's through breast milk or formula!