This will be no help.
I can never guess a birds sex until it is much older than 3.5 weeks. It takes longer for my eye to detect the difference in feathering.
Keep in mind that I raise mutts. Sexing your birds gets a little easier if you are very familiar with the characteristics of your breed as adults.
One thing I have found that is a pretty good indicator of sex, at least with my mutts, is the size of the leg bone. Comb size can be deceiving. But it has been my experience that roosters always have bigger bone to their leg. In some instances, markedly heavier bone. Even if they all hatch looking the same, within a month little roosters have bigger leg bones, and hens have slightly shorter more delicate legs.
Of course, this can be tricky too. A chunky hen can have legs as big as a light boned rooster. This is made worse in my case since I have so many mixed up genetics in my flock. But if you have ALL RIRs, for example, you know they should be pretty uniform, therefore larger leg bone is most likely an indicator of a rooster.
I rely on bone size more than comb development. BUt to truly tell you have to wait a lot longer, at least I do.