Usually LCs recommend to start introducing practice bottles by usually like week 3-4 and then the KEY and let me emphasize this again THE KEY is to not stop. Giving a bottle at 3 weeks and they take it easily and then say oh great he's fine with bottles is a mistake many of us have made. At 3 weeks they still have a suck reflex and are likely to take the bottle but once that goes away a few weeks later unless you're doing it every day or every other day or so, some babies will stop being interested and it can be so tough.