I think you have 2 pullets and 1 cockerel mainly because one's comb is more pink. Plus, it looks like in the picture to me that the one with the pink-er comb is less feathered out.
Once upon a time I had quite a few RIRs (before the neighbor's dog came around) and I could tell easily at around two weeks because my pullets had big time wing and tail feathers and the cockerels had more prominent combs.
However, I can see that you are saying that they are all looking the same to you. Usually I can tell more by looking straight down on the chick, instead of from a side view.
Sorry...it is so hard with pictures...wish I could see them in person.