"adopted three new hens from a friend..."
This is most likely the culprit if you're certain that molting is not the issue. Chickens hate change, and changes can put a whammy on egg production. I doubt it's the issue, but do you pay attention to their poop? Look for evidence (or lack thereof) of worms, just in case. Worms puts a big whammy on egg production as well.
Now, another consideration is simply that the days have grown quite noticeable shorter (at least in my area) over the past few weeks, which often impacts egg production. But my money is on the addition of the newbies...