If nothing else, you probably want to arrange something so that you are only going to be heating a small area; I don't know if you've worked out the economics on a 250w bulb but it makes a noticeable dent in your hydro bill, and isn't really likely to be necessary for just 3 chickens (you can give them a smaller partitioned off area to put a lower-wattage bulb in, if they need anything at all).
At the moment I would say that you are probalby best off keeping the lamp turned OFF, let them get used to cold. It sounds like they are happy enough.
There are a buncha other things you can do to minimize how cold your coop gets (see link in my .sig) so if you worry about your babies
it might be worth seeing how well you can winterize the coop in that way, too.
Good luck, have fun,
Pat
At the moment I would say that you are probalby best off keeping the lamp turned OFF, let them get used to cold. It sounds like they are happy enough.
There are a buncha other things you can do to minimize how cold your coop gets (see link in my .sig) so if you worry about your babies
Good luck, have fun,
Pat