Your chickens are really too young to sex, in my opinion. I thought the black one was a cockerel because its comb seems so large for that age. But you are better off and will get better answers if you wait until your birds are feathered out, at about 7 weeks old. Look for larger, red combs at...
Yes, it looks like both the black bird and the naked neck have pea combs. So the black one cannot be an Australorp. I do think it is a cockerel due to comb size. I think the naked neck is a pullet.