You have no idea how hard it was seep rating this 3 week old from week olds... I think it's a pullet cause it's got motherly instincts toward these smaller chicks
I don't have experience with marans,, buff Orpingtons,, or black australorps. So I'm not going to try and guess sex at this point. Somebody who is more familiar with those breeds will prob. be more helpful there. Now if you wait another 6 to 8 weeks and take new pics and post them it will be easier to identify sex based on feather development, color and wattle/comb development.
By the way, the 2 golden/red/white colored pullets in my Avatar pic are my 2 Red Sex Links when they were 5 months old. Yours will probably look similar.