Hi Henry - in my humble opinion, three week old chicks can go outside on a sunny day in 50 degree weather. Make sure they are in sunshine and just watch them. If they are running around and playing, as I suspect they will, they are fine. If they start huddling together, which I've never personally seen my chicks do in cool sunny weather, then bring them in. As far as being outside at night in 30s - if there is heat provided and no drafts they'll be warm. Just make sure they are secure and safe if you are using a tractor and if you cover it in plastic and have heat lamps, they'll be nice and toasty. I had that first batch that started this thread out in that hutch in 30s and draped plastic over it. Nearly baked them alive one night when, for extra warmth I put a heavy blanket over plastic. When I lifted the plastic the next morning I could feel the blast of heat that had nowhere to escape and it felt like an Easy Bake Oven in their hutch and they had drank all their water. So go easy on the winterizing.
Right now I have baby ducks, two weeks old, who are sleeping outside, on the ground with nothing and some nights have gotten down to 40s. I was originally trying to catch them and put them in duck house where I had a heat lamp but they would not stay in and kept running out. I finally decided that if they got cold enough they could go in on their own but they seem to like it outside which is where the big ones always sleep. One night I had DH "help" me because it was supposed to get to low 30s and we caught all the baby ducks and put them in a kennel with a light inside the duck house. Next morning we discovered that all the smallest ones had escaped through the gaps at bottom of hutch. Only the larger and older babies were still in hutch. So now I don't even bother with leaving a light on in coop.