Pullets do not turn red until they are close to being ready to lay eggs. A chick that is still feathering in is nowhere near being ready to lay eggs.
It's a boy. Only boys will turn that red, that early.
At 3 weeks, I'm going to say rooster too. But I'd also wait a couple more weeks to be 100%. I've seen some pullets develop big, red combs early...
This is Rosie. Most people were convinced she was a rooster due to her large comb at an early age.