At 3 weeks even layers won't fly out, and trust me, meat birds aren't overly inclined to go anywhere. I've never had any of my Cornish/Rocks get more than a foot or two off the ground.
What breed are you raising? If it's one of the heritage breeds, or a dual purpose, they might be able to get...
Yes, they will absolutely be fine as long as the temperature is above freezing. My layers chicks are always fine outdoors after 3 weeks (over 32 F), and meat birds are quite cold hardy, and do not do well at all in the heat.
Last year we had meat bird chicks in the middle of the summer, and I...