At that age temperature sure is not a problem. I’ve had chicks less than half their age in an unheated coop when it got below freezing. No problems at all. But predators might be a problem. I don’t know if predators are a concern for you or not.
Three weeks is a long time to have to put them in. I’ve had a few be stubborn like that but always at least some go in the coop pretty easily. I do have a lot more chicks than you though. As few as you have, averages don’t mean a whole lot.
Chicks that are not yet roosting usually sleep together in a huddle, often on a corner or protected place and usually fairly low where they consider themselves “hidden”. When you open the coop and let them in the run, they may think a place in the run is a safe place to spend the night. I go through what you are talking about practically every time I open up the grow-out coop to the run. Even though they may have been in that grow-out coop a week or more, I usually have to put some into the coop after dark. Sometimes just doing it once is enough for them to catch on, but some have lasted a couple of weeks. I think part of the problem is that my grow-out coop is elevated and they like to sleep lower down until they start to roost.
Most of mine tend to start roosting about the age yours are. When they start roosting, they should go in on their own. Before they roost, I usually do get them to sleep on the floor inside the coop,
Another potential problem is that chickens cannot see too well in the dark. Sometimes the insides of the coop get dark earlier than the outside so the chicks don’t realize it’s time to go to bed until it’s too dark to see inside the coop. Some people have solved this by adding a small light inside the coop on a timer so they can see to go to bed, or a better option in my opinion, add a window.
