Aw bless him, 3 weeks is still a very short time, keep feeding him to build up the bond. Before you approach the cage, tell him what you are going to do, “I’m just going to walk toward you” to give you this treat” show him the treat and take one step at a time and see how close you get before he stiffens/feathers change/turns and walks away.. at that point you turn and walk a step or two away, too. Show him you respect his decision and messages. Try again a couple of minutes later once he has softened back, building on this over a week or so you should be able to get closer without him walking away. 4/5 times am/pm then relax.
Keep a routine so he can build trust in that too. As well as hand feeding him, you could near read to him, play cards or chess so he can watch with you.
Some birds take months to accept their new surroundings and caretakers. It’s a big upheaval of his world, new place, cage, people routine, food etc.
My boy didn’t walk away but wouldn’t come willingly over, unless I had food so I would feed him 10/20 apple cubes through his cage, then walk away. After a while he made the decision to came over to greet me as I walked to him.
