I too have a three week old....I am personally not producing enough breast milk for her, so I am supplementing with Similac formula. But typically a three week old will eat 2-4 ounces at a time....more typically 2 ounces though, if they get more than that then they tend to spit up more. I believe at 3 weeks Noah was drinking around 4 oz. of formula. I always burped him after every couple of ounces at that age.