I don't think they are over 3 weeks. They were probably a few days old when you got them. You could call and find out when the shipment arrived. They were probably 2 or 3 days the day they got them.
Looks like they are still inside your house? Watch out Cornish's get stinky much fast then standard chickys! If possible I would move them into the garage or garden shed or even the neighbors house.
Mine are 4 1/2 weeks, and in a shed as it is really cold. I never thought of the nieghbors house or garage to aviod chicken dust and cornish X. Great suggestion.
Yep they are in our house. I change the bedding (shredded paper) very often. That's partially why I was asking about their age. I can't wait until they are old enough to be moved into the woodshed coop. I refuse to use a heat lamp in there for safety reasons though so they need to be able to keep themselves warm out there. Maybe next week.....