They can go out any time. The questions are:
Can they stay warm enough? A heat tray is the perfect solution. I got the one from Premier1 with the cover and they did fantastic. I didn't have to worry about fire or about them not thriving because of being chilled. One morning DH went out to check on them there in the storage room next to the coop, and they weren't under the tray: they were huddling on an empty bag that I had used to cover the feed buckets. So I took the tray out. I don't think 3.5 weeks in your temperatures is ready, but what do I know? I put mine out at 4.5 weeks with the tray until they were a little over 5 weeks old. Yesterday they were 6 weeks old, and tonight is their first night up on the roost.
Are they safe? Depends on whether the coop can keep out predators. Just a warning, lots of people on here have suffered tragedies from heat lamp fires. That's a cheap but dangerous way of providing extra heat.
You could put yours out and see what happens, but I'd wait another week.
