Hi Turkey12345...
So, I have found that everyone has different opinions on housing. Some people put their turkeys in coops every single night (some places have a lot of predators that will go after poultry and eggs) and let them free-range throughout the day, some people seem to do it seasonally or based on the weather (only put their turkeys inside during the winter or otherwise bad weather), and some people just let their turkeys completely free range and roost in trees at night. I think most people I know who have let their birds roost in trees at night have lost a bird here and there but were ok with that because of the number of birds they had, the convenience, and the birds seeming to prefer it. My birds ALWAYS let themselves into their coop at night when the weather is bad/cold, so I personally feel like I ought to provide them with that option. During these warm days/nights when they stay out in their enclosure and roost on their branches - I just let them.
Anyhow - as far as your actual question about the young poults... once they're old enough I'm sure roosting on the barns will be just as good as roosting in trees and they'd probably like it... but 3 weeks old seems quite small to have them outside all of the time. My poults are 6 weeks old right now and still small enough to be easily killed by predators. Hawks have been known to swoop down and kill a poult, even if it was too large for them to carry away. And a neighbors cat attacked one of the poults from my last clutch when it was about 5 weeks old and would have killed it if I wasn't out there with it. I don't let them free range without my supervision until they're older. I've never let my birds free range completely and roost in trees at night, so maybe someone else who does could give you their opinion - but it only makes sense to me that if they're too small/young to fly away efficiently then they're too young to be out alone all the time, especially at night without shelter. 3 weeks old is really quite small and I would recommend giving them shelter initially. At that age, they may even still need a heating lamp on at night, depending on where you live and how cold and/or damp it gets!
