Are you talking about chickens or ducks, here? Chickens you can start picking out roosters by their combs at a pretty young age as well as things like stocky feet, etc.
For ducks, male is called a drake. 3 weeks is a bit early to tell, but soon some of your early blooming females will start quacking. If they quack, they're girls. Personally, I've got four ducks that are sexed to be females... they are four weeks old, three we've heard some "peep-peep-peep-QUACK" from, the fourth is hopefully a girl too but is just the smallest and perhaps is going to take a little longer to quack.
Once they are fully feathered, and there are ducks who aren't quacking, they're probably drakes. Drakes don't quack like females, but they do make a hissing noise or a deeper quack noise. You can look online for audios of the sound. Drakes will also develop a little curly fail feather.
