Theres a relatively easy way to tell the difference between male and female chickens at about three to four months of age (sometimes earlier, sometimes not.) Three weeks might be too early, but you can try this.
Sit or stand with a young bird facing your stomach. Look down at the area which on a human would be the lower back (this is called the saddle), just before the tail. Saddle feathers on a young male bird will be glossy and have pointed tips. Saddle feathers on a young female will be rounded and less glossy.
Of course, you can also look at comb size and length, as well as wattles (males get bigger redder combs and wattles than females.) As well, males tend to stand taller, have longer and thicker legs, and become more alert during potential danger, while females tend to be shorter, rounder, and crouch down during potential danger.
Hope this helps