Soo, since it's 42 and they're voluntarily open to the elements instead of inside the coop - I think they'll be just fine at night when it's 35 =) Sometimes I have to wonder if keeping chickens out of the cold until they're 6-8 weeks does more harm than good. These ones were living at 60 degrees from week 3 onward and never did they act like they were remotely cold.
I put my 4 week olds out in the coop (in a barn) in mid July with their heat lamp hanging down in front of a LARGE cardboard box on its side with the opening facing the corner. It was connected to the "reptile" remote sensing thermostat at it's lowest level. I had the thermostat plugged in at night for the first week only (it was in the 50s F). Being new to chickens I was, of course, afraid they would be too cold. The heat setup is what I used for their brooder (bathtub in the house).
If yours were at 60F on week 3, I think they will be fine. I went with the "95F for a week, dropping 5F every week until they are 4 weeks old" thing. Dang was it uncomfortable for we humans in that bathroom!
They are now 24 weeks and I have no intention of heating their coop. It was 27F and breezy yesterday. They spent a fair amount of time in the barn alley but they also went out and ate grass or whatever with their feathers blowing forward when the wind was at their backs. Didn't even seem to notice.
I agree that keeping them warm too long into the fall could be detrimental. They need to feather up for the cold weather.
Bruce