I'm concerned that you've had your Cockatiel for 3 years now and he's only been let out of his cage once!!!!! That's not good at all, he's probably extremely bored, neurotic, and totally unstimulated..
All birds are going to fly around the room like that when they are first let out, especially a bird that was not hand-raised...But you cannot have a bird locked inside of a cage for the first 3 years of it's life, that's not healthy at all for him, and he's likely plucking out of boredom, frustration, lonliness, anxiety, etc...
It's obvious that your bird was not a hand-raised baby, he came from a pet shop who buys their birds from large vendors, and they are parent-raised birds. So ideally you would have started working with your bird every single day from the week your first brought him home until now, and he'd be a very tame, loving little bird...But he's been locked inside a cage for 3 years!!! You have to understand, he's got the intelligence of a 3-4 year-old human child! So he must be mentally stimulated and not be lonely, and he must be allowed to come out of that cage, as he's a bird! They fly, they need lots and lots of exercise every day to stay healthy, both mentally and physically.
***I'm going to assume that you've never had him for a Wellness-Exam in his first 3 years either, correct???
****Here's what I would do if I were you: You need to either start letting him out of his cage every single day for at least a few hours and trying to find a way to stimulate his mind/pay attention to him, or you need to re-home him to someone who will. He's plucking himself when you turn away from him because he is bored to death and extremely lonely...And no, please do not "bring home a friend for him", because that is not the answer at all and you have no way of knowing if they'd even like each other or be able to be near each other, so that won't work and is not the answer...
***If you want to keep this bird, then the best thing that you can do is to:
#1) Locate the closest Certified Avian Vet or Avian Specialist Vet to you (We can help you with this, there is a great search tool on this forum for finding an Avian Specialist worldwide, and that way you won't accidentally take him to an "Exotics" Vet that doesn't know what they're doing or talking about
#2)He needs a complete Wellness-Exam, as he's now 3 years-old, hes now plucking, and he's never seen a doctor in his life! He needs to at the very least have a complete visual/physical exam, a Fecal culture/gram-stain done, and routine Blood-Work to make sure he's not suffering from any of the many Avian Viruses that could be causing him issues...You also need to see how his Liver and Kidney functions are, because if you only have fed him an all-seed-mix diet, then he's probably suffering from Fatty Liver Disease, among other issues.
#3) There will be people who disagree with me here, but I think in your situation, which is quite dire in-regards to your bird's physical and mental health, and because I think this is the only way that you're going to even attempt to let him out of his cage every day and try to work with him every day on taming, stepping-up, etc., and just allowing him to hang-out in your house outside of his cage, I think you very much need to have the Avian Vet clip his wings. This will keep him from flying all over the place frantically so that you won't be scared, but more-importantly it will allow you to open up his cage every single day and then just allow him to come out in his own time. Then you can make some progress, and he can have some exercise, mental stimulation, etc. Because his first 3 years of life being locked inside a cage have not been good...
I'm not blaming you or telling this to make you feel badly, but rather to impress upon you how important it is for a parrot to get at least 4-5 hours of out-of-cage-time every single day! With the intelligence of your Cockatiel and the body that he has, which is made for flying 10 miles or more every single day, I'm honestly surprised that he's not completely plucked and also self-mutilating, because usually in situations like this, parrots are not only plucked bald (everywhere but their heads/necks where they can't reach) but they also typically have deep, open wounds, usually in their chest/belly area, where they have literally chewed through their flesh...Imagine if you took a human child who is 3-4 years old and you locked them in their play-pen 24 hours a day, 7 days a week, for the first 3 years of their life...Imagine what they would be acting like...That's essentially what happens when a parrot is locked inside of a cage 24/7 with no interaction at all...So you need to do something about this situation NOW, regardless of what you decide to do, it needs to be ASAP...If I were you, I would at least try getting his wings clipped ASAP, and then committing to working with him every single day, without fail, and letting him have at least 4 hours of out-of-cage-time every single day. It may take him a while to build-up the courage to wonder-out of his cage once he realizes that the door is actually opened, and that's fine, you have to let him do it at HIS PACE, NOT YOUR PACE...
This will be a marathon, not a sprint. It could take months and months just to get this poor little guy to start regularly coming out of his cage when you open the door, let alone stepping-up for you. But you need to do something, because the way he's been living is not fair to him, and also not at all healthy for him, neither physically or mentally...And the other issue he's most-likely suffering with, almost certainly, is Fatty Liver Disease, simply because he's on a seed-mix staple diet instead of a pellet staple diet, and he's gotten absolutely no exercise throughout his entire 3-years of life,
so the combination of him eating a very unhealthy, fatty diet every day along with not getting a bit of exercise at all, ever, has most-certainly caused him some potentially serious health issues. HOWEVER, THE GOOD NEWS IS THAT THIS IS ALL REVERSIBLE AND YOU CAN HELP HIM TREMENDOUSLY!
****It's obvious that you care a great amount about your Cockatiel, otherwise you wouldn't have come here, made an account, and then asked us for help... We can help you through this, every step of the way...But my personal and professional opinion with this particular situation is that you're not going to be able to start working with your bird, nor even be able to give him any out-of-cage-time at all as long as he can fly...It's extremely difficult to work with a non-tame bird when you're constantly chasing them around the room, from window to window...This only serves to scare them more and more, and it actually takes your progress backwards...That's why I think the first thing that you need to do is find an Avian Vet, take him for a full Wellness-Exam, and have them clip his wings so that you can start getting him out of that cage right away.[B/] Once you get his wings clipped, then we can help you step-by-step with the taming/training process, but just being able to let him come out for exercise, interaction, so he can just do some exploring, just so he can simply start having a life, this is going to get the process moving...Otherwise his plucking is only going to keep getting worse, and this is going to end very, very badly...When given love and affection, exercise, and mental stimulation every day, along with a healthy, low-fat diet,
Cockatiels should live into their late-teens/early-20's. So you can have a lot of good years with your baby boy ahead of you, but you have to start-over...
***If you want help finding a CAV/Avian Specialist closest to you, just let us know what city/state or country you live in, and we can do a search for you, or we can find the link to the tool for you...Very important is that you make sure that if you do go ahead and get your bird's wings clipped, which again I think is the only way you're going to be able to start helping your bird, that you not only have it done by ONLY an Avian Vet (not an Exotics Vet or a Pet Shop!!!), but that you specifically request that they do it "Conservatively", because your bird needs to still be able to glide across a room and glide gently and with control to the floor, and that the wing-clip only keeps him from gaining any altitude. If they don't know what they are doing it ALWAYS ends badly, and the bird ends-up dropping like a stone to the ground, bleeds, has no control at all, etc. So it's necessary that you request/demand that the CAV/Avian Specialist Vet "Only clip the outermost 5-6 Primary Flight-Feathers on BOTH WINGS". It's very important that you word it that way, because they should not ever clip into the secondary flight-feathers, that's horrible, and they should always clip BOTH wings...Some old-time bird breeders still think it's okay to clip only one wing, but its a horribly cruel and dangerous thing to do, it causes them to be completely off-balance and it causes spinal damage...So, again, "Only clip the outermost 5-6 Primary Flight Feathers on BOTH wings"...