How big is your coop and do you have a run associated with it? Is the combination coop/run big enough to leave the chickens locked in there for a week or so? If so, you can try to get the chickens out of the tree after dark and lock them in the coop/run for about a week. Have the nests in the coop with fake eggs in them. They might or might not start sleeping in the coop when you eventually let them go but they will at least have seen the nests and fake eggs. That might or might not be enough to get them to use those nests.
If you do that, feed them in the coop and keep feed in the coop after you let them out. That should get them to going in and out of the coop during the day. Again that might be enough.
There is no telling when they will start to lay. It could be in another three weeks or it could be months. There is no sure way to be able to tell when they are about to lay, especially if you can’t catch them and physically examine them. A clue is when the comb/wattles turn bright red but that’s only a clue and it does not always work. It would be nice if you could time locking them in the coop with when they start to lay but that’s a lot easier said than done.
You could try building some nests sheltered from the weather near or under that apple tree but that does not come with a guarantee either.
I’m assuming they are pretty flighty, scared of you? Do you feed them treats? Take a container, a bucket or a bowl, and start feeding them treats from that bucket or bowl a couple of times a day. Give the same call, something like “here chicky chick”, whenever you feed them these treats and rattle that bucket. After a while they will come running whenever they see that bucket or here your call. You may be able to lure them into the coop or run and lock them in there. Or at least get them used to getting their treat in the coop so they go in there and hang out near the nests.
They are really good at hiding nests. I’m afraid you may be in for some challenges. Good luck!
