I would like to reassure you that what you are describing is absolutely normal. That's good to know of course, but it doesn't make it any easier to deal with when you're in the situation.
I am so sorry that you lost your dad at such a young age. It must have been devastating for you and of course still is a huge loss that must leave you feeling very sad and maybe also angry at times. No wonder you have lots of emotions within you that are bursting to get out. It is usually NOT a good idea to keep holding in your feelings. They only tend to get bigger and stronger and then keeps coming out in the wrong ways - as you are describing with shouting at your mum and her boyfriend.
It would be good if you could find other ways of letting your emotions come out so that you can deal with them in a better way and so that you can work through your grief. You will never get over losing your dad and that is not the intention either. Many think that after a year - or as in your case after three years - they should be 'getting over it' and cope with their loss. This is not the case - you don't get over losing a parent - you just learn to live with it and yes, gradually the grief becomes less painful and life goes on in a different way than before. But, you have every right to feel sad and angry that he's gone and you need to talk to someone about how you feel. You may think that you are holding your feelings in, but of course you are not. Your feelings ARE coming out, but not in a way that is good for you or anyone else. You are just letting off steam that is boiling over, but it doesn't really help you in the long run.
I think you have realised this now and it is very good that you have taken this step and have written to us about it. It's a start in talking to someone about how you feel. This in itself may help you to a certain extent, but I recommend that you find someone near you that you trust and that you can talk to face to face. The most obvious thing would be to talk to your mum. You see, children are often very good at hiding their feelings to their parents and this often results in many parents not realising what their children are going through and how hard it is for them. Your mum may not understand that your current behaviour has got something to do with the death of your dad 3 years ago. If you have been good at pretending everything is okay, she may just think that your behaviour is due to you becoming a teenager.
So please try and talk to your mum - or at least to someone else that you know and trust. There may be others in the family you feel better talking to, or maybe a family friend, a parent of a friend or maybe your teacher?
You need to start talking about what happened three years ago and work through how this is affecting you now. This way, you can learn to live with the loss of your dad and stop pretending on the surface that everything is okay. I hope you get the support you need and please do get back to us again if you think it can help you to stay in touch. Remember that you can also use the forum on the riprap site to talk to others in a similar situation and who will be able to understand what you are going through.
Good luck and take care.