This thread is 10 years old, OP is no longer on BYCSpace requirements for coop (overnight sleeping housing): 4 square feet per chicken
Space requirements for run (outside daytime living space): 10 square feet per chicken
These are just general recommendations and MORE space is *always* better. Crowded chickens can get nasty with each other.
But you have 3 hens and that is small but should be good alright.
Where are you located in the world?
Typically ventilation is above the roosting bars, so noxious ammonia fumes can travel UP and OUT and wind doesn't blow them from the side -- if you live in a cold or cold-ish environment.
If you live in a hot-as-heck or mild environment, they don't even need anything other than rain cover, as many open-air coops exhibit.