I see the pen already has a roof. Maybe add a windbreak on one or two sides of the pen. That should be all the need.
Chickens that have protection from rain/snow and from wind can do just fine in temperatures well below freezing (32 degrees Fahrenheit), so if your temperatures are staying above 40 I do not think there is any reason to worry.
You said the chicks are three months old: at that age, they should be fine with the same conditions the adults need.
I would probably leave quite a bit more area open for ventilation than you did.