Sounds like you have your hands full! It must be exciting to have all those birds live with you. I know I enjoy having my parrots around me.
From your post, it seems like your grey Indian Ringneck has bonded to your father. This is typical behavior if the bird is matured. Does he have his ring? If so, he is in love with your father. This became apparent to me as you stated he tries to attack anyone who removes him from your father. He finds your father quite attractive and safe.
So how can you change his behavior? The important thing is to have your father start a positive reinforcement program with him. He should start rewarding him for behavior that he wants and ignore behavior that is unwanted. Overtime, the ringneck will come to understand what is acceptable and what is not acceptable within your home.
You have to know that it takes time. These birds are stubborn and it can take quite a while to change any behavior. You can also help to nurture your relationship with your grey ringneck by doing activities he enjoys with you. Does he have a favorite treat? Does he like to run around on the floor? Does he like to sit upon your shoulder? If so, find an activity that he likes, with you, and this will help to show him that you mean to strengthen your bond with him. The key here is patients in finding what will open his eyes so to speak to accept you as a wonderful companion.
I know you had said all your other ringnecks gang up on your other birds, correct? It was hard for me to quite understand what you meant. But if this is true, you should start by keeping them in separate cages close together. Interact with them altogether and reward them altogether. It is important you phase this behavior out over time. Reward them for being close together. I know it sounds like beating a dead horse but the key is to reward them for behavior that is good.
Please do a search on our forms as we have tons of information about this sort of stuff.
Please keep us updated and congratulations on your three new family members!