On these ducks, this looks like the start of drake coloration growing in. If they are all the same age, I'm guessing the one you originally asked about is female. You should be able to voice sex them by this age too. As long as they are no longer making baby peep noises, males and females will sound different.
They all look like drakes to me. I am basing it on the color of the feathers above the tail. Males have solid, dark feathers. Females have lighter, two-toned ones.