I'd like to say it gets better, I suppose it does a bit, but I'm in the same situation after 5 years.
Talk to someone. Anyone who says you shouldn't be sad because it's been 3 years isnt worth your time. Death affects everyone differently. And it never leaves you.
I'm sure your mum will be having similar feelings, try speaking to her?
Your dad would want you to succeed, just try and do your revision with him in mind positively inspiring you and leading you on your path to do well.