Can a 3 week old baby overfeed?

Yes and no: By nature an infant will enjoy suckling and can feed continuously if offered. In general more than 15 min/side when breastfeeding has emptied the system & any more is just nurturing. Bottle feeders generally stop when the bottle is empty & a goal is to put enough in until a little is left when they stop. If they spit up often, too much. Usual feed=1/2 their pound weight as ounces. (8lb=4oz)