Hi and welcome
There is a reason why the egg factory sell them at this age. It is because they are about to go into moult and stop laying. It is not economic to keep them during this non laying period even though there are usually many more years of egg laying left in the hen. They get the most productive 18months out of them and then replace with new pullets. Most just go for meat (not that there is much on them) so it's great that you have given these girls a home and they will almost certainly reward you in due course, if not now, like two of them.
As others have said the change of environment is a big shock to their system, so that will also play into it, but it may be that this bird has already started her moult whereas the other two have yet to start. I imagine it may be difficult to tell that they are moulting with factory chickens as they often pluck their own feathers out of boredom but there is a seasonal cycle to it and it will probably take a couple of months for them to come back into production once they start, So don't be too despondent if the other two stop laying as well. They should eventually all grow beautiful new feathers and recommence laying you lots of eggs, but it may take more than a couple of weeks, so be patient.
Regards
Barbara
