Looking at comb and wattle development mostly. Cockerels would have massive combs at this age. Yours are all still very small. 12 weeks old is when cockerels start getting male feathers, but I don't see any.
That one looks like a cockerel. At this age you are looking for comb and wattle growth. Any chicks with large bright red wattles and combs are cockerels. Females will have small pale combs and wattles. Looking at the saddle area isn't going to be an indicator right now. Male specific feathering...