3 year old toddler i don't even know anymore?

My nephew, i live hima dn he is starting to be a very bad kid. We've tried spanking punishments, time outs and stuff like that. He does things after we tell him he can't and then he also is really angry at times like slamming doors and hitting people and throwing things across the room. None of my family does this so i don't know how he learned to. He won't eat anything except macaroni and cheese adn chicken nuggets and SWEETS and JUNK FOOD. This isn't healthy for him adn i know that but please i just need help it is affecting his life and others.