At that age, those marans were probably obvious. Some cockerels show early... some don't.
For instance... This is a cockerel I bought from a breeder via the internet. I picked him out, on purpose. He's about 4 weeks in this pic, and obviously a boy looking at his comb. That's w/out taking...
agreed, three cockerels. Which is better then you thought, and the risk you take with buying straight run.
Keep in mind... chest bumping is perfectly normal for chickens of both genders. Its how they establish dominance and the pecking order.