Possible things to consider: only let higher ranked, mature hens hatch out (as they're less likely to put up with any possible bullying of themselves and their brood) and integrate the hen and babies back into the flock much earlier, when the hen's protective instincts are at their peak.
Hmm, sorry to hear about that. It's possible that if the mother wasn't diligent enough in protecting it or the other hen was aggressive enough that it could've killed it, because the chick was still new to the flock and also the weakest of the three. Really it should be up to the mother to...