You mentioned that the run is totally enclosed, meaning a wire over the top too? Still, depending on what size wire you used a mink could still gain entry and kill the three. Doesn't seem likely that they were killed by the older hens considering the age of those killed. Be on guard tho, if it was a predator of some sort it will most likely be back unless it was just passing thru. So sorry for your loss, now to start all over again...