It could be that only one hen is laying an egg a day, but could it not also be that all three hens are laying one egg every three days as they slow down?
It is usual to buy in some younger hens every now and then, never one or she will be picked on, but at least two to keep production going as your first birds get older.
We keep our older hens as well, even if they don't appear to be laying any more, but they usually pop out a surprise egg once in a while, and tend to pass on their wisdom to the younger hens.
I agree that the heat is probably not helping - our nine hens are laying a couple of eggs a day and we suspect someone is laying away in a secret spot we haven't found yet, with a view to raising chicks.
With hen keeping, you can either go the intensive route and work your birds to death, with lighting and heavy laying, or you can go the more natural route, keeping birds which stop laying in the autumn while they moult and build themselves up again for more eggs in the spring, unstressed and healthy. I know which route I prefer