For hens, a couple of hours away will probably not affect the pecking order. If they are acting weak because of illness that could have just as much or more of an effect on pecking order. Each flock is different, but I have removed a hen for a couple of days due to a leg injury and she was not harassed when reintroduced. The fact that you are removing 3 at the same time may also reduce the stress of reintroduction for them. They will definitely remember each other - chickens are good at recognizing and remembering individuals.