Can you put them so there is just a wire mesh fence separating them? That way they will be together but not able to peck, and that will reduce any problem of putting them back together again. If the pecked hen has any red showing from the attack, then both of the other hens will tend to peck at that - it's what they do, not that they are evil. So keep them apart until she is well healed up then re-introduce them at night as Doganjo says, but keep a close eye on them. Make sure they have plenty of other things to distract them, such as brassica plants hung for them to peck.
It could be that the Sussex had a small injury to her vent, perhaps after laying her egg, which would have started the Black Rock pecking at her, and the redder it got the more she would peck.
Someone has to be at the bottom of the pecking order and with only three hens it will be obvious who it is
