how old before I move my chicks to there own coop

We had a light fall last year, luckily we caught it before it started a fire. We brood in a big stock tank and keep the light clamped/nailed to a 2x4 now. It scared me half to death when that one fell.

I have 3 chicks that are 3 weeks and 3 days old. They have feathers on about 3/4 of their body. It has been in the 70's and 80's here during the day, going down to the low 60' at night. I have them separated in half of the regular chicken coop, which is about 4'x10'. Everything I read is for a larger group of chicks. I have a heat lamp on them at night and know that the older ladies in the other half (with an open door) must be hot. How soon can I remove the heat lamp at night? The bedding is pine 3" shavings.

Any Ideas. Thanks.

so far knocking on wood,when I let my out to forage they have been safe,i am in the house but can hear them if they start "talking" well this has only happened once I went out to see what was going on and saw nothing...whew...yes mine are way to spoiled that is just the way it has been their whole life,yes some mornings when I have to work and I get up at 5,it can be a hassle getting their food and water together,as well as my coffee and lunch,but I tell you I believe it is paying off everyone that gets my eggs tell me how beautiful brown they are and that they have never seen an egg look and taste so good...and they are just pullet eggs,praying next year they will be bigger brown beauties well my chicks should ship tomorrow and I am so ready for them to get here more so to calm my nerves and make sure they are healthy and warm,i will post pics of them and their progress I hope when the time comes for them to meet the other chickens everything goes well and they get the peeking thing over with fast I just want everyone to get along and be happy...and of course lay lots of those beautiful eggs,i have a lot of people ready for when they start laying