As already mentioned, this process is just going to take time and patience, especially if he wasn't a hand-raised baby or he was but he stopped getting any human-interaction after he fully-weaned, which is quite common, especially if you bought him at a pet store like Petco. They sell "Hand-Fed" Green Cheeks and Sun Conures that were actually Hand-Fed, but they unfortunately stop getting any handling or human-interaction at all after they wean, and depending on the individual store they may or may not get attention from employess on a regular basis...So it's all about time, patience, and giving him as much time and attention as you can each day by simply being in the room with him, sitting next to his cage and reading to him, talking to him, etc.
As Allee mentioned, it would be great if you started to let him out of his cage every day by simply opening up the door to his cage after putting it in a room that is safe and secure, and then just waiting and allowing him to come out in his own time. He may not want to come out and fly around right away, or he might come right out, you don't know, but you never want to try to force him to do anything...Make sure you have his cage in a room with a door that latches, carpeting on the floor is best, cover all of the windows up (as well as mirrors), and then just let him come out in his own time. Start doing this each day so that he not only gets better at flying, but also so that he's getting some exercise each day, and this should also aide in you bonding with him gradually over time.
Also, it's very important that you have his cage located in the "main roon' of your house (except for when you're allowing him to have out-of-cage-time each day in a safe, secure room)...The "main room" of your home is the room where you and anyone else who lives in yoru home (his "Flock") spend most of your time whenever you're at home. Usually this is the Living Room, TV Room, Family Room, etc. For birds who are not used to human-interaction or birds who were originally hand-raised but then were not given any human attention after they weaned, it's extremely important that they are simply around and among their "Flock" as much as possible. So whenever you and other who live with you are watching TV, playing Video Games/other Games, Reading, Talking, Eating Meals, on the Computer, etc., and where visitors spend time when they come over;, just having him in the same room with everyone is going to help tremendously with getting him more and more comfortable being with people...Plus, whenever people put their bird's cage in a back room of the home, or a bedroom away from where the action is and where people spend their time, it tends to also cause a lot of screaming and results in the bird not wanting to entertain themselves inside of their cages with their toys because they can hear that people are home, but they can't see them and they want to be with them (they are "Flock Animals", so they need to be among their Flock)...
3 days is like 5 seconds in "new-bird time", so it's just going to be a gradual process. But just keep doing what you're doing, try to include him in everything you can, and give him the opportunity to come out of his cage every day in a safe room for exercise and bonding time...
