Depending upon the line of Rhodebars, the chick in the middle is likely a pullet, just a lighter colored one than the others. Males should have a white dot on the head, and no chipmunk bars but more of a murky brown/grey. Sometimes you get wheaten throw backs, which fortunately you are not seeing. Wheatens can be 50/50 male/female and the head dot is so faint you generally can't see it.
Rhodebar autosexing is not infallible, and if you are breeding, you should "cull" any that are not clearly autosexing, per those who breed for longevity.
LofMc