Here are a few pictures of some 3 week old chicks. One withe the bigger comb is also the biggest chick, I think it may be a roo. The smaller has a smaller comb. The bigger chick is Elvira, smaller is Lilly. Elvira may become a Leon...LOL
It's a bit young to tell, post again in a few weeks and maybe we can have better guesses. I usually can't tell on some of my CC roo's till they are about 5 weeks as some are late bloomers.
I saw a tv show that said to sex them check the wings. If the feathers are in an even row it's a pullet if they are uneven it is a cockeral. They were at a hatchery showing how chicks are prepared for shipping.Also, if the comb has any pink to it it is usually a male. Hope this helps.
Are those a Cornish Cross type breed, or a white leghorn or other white egg layer kind? I can see obvious wattles on the one chick in the very last picture. I can tell on the other one, but from these photos my guess is that the chick in the last photo is a roo. He looks just like some of my Cornish at 3 weeks old...although he has more feathers. Is there any pink at all in either of their combs? If they have any pink/red in them at this age they are almost certainly a roo. Hope that helps!