The can and do drink from a bowl at 3 days hatch so it might not be understanding what you're doing. To keep down on mess, if you have an empty sour cream container or something like it you can cut a hole in the lid big enough for it to put its head in completely (it will need to wash its eyes and nares...nostrils) so that it can drink and clean. Cutting up a small amount(like a pinch) of greens, untreated grass even, into teeny bits and floating it on the water helps stimulate their drinking as they try to get the greens. Unmedicated chick crumble is what you're going to want to feed. Very gently dipping the bill into the water once usually learns them where their drink is, a couple times sometimes needed. But a 3 day old duckling will eat and drink on its own if it's not poorly so a 3 week old would definitely be able to do so.