Abbreviation of "nah, nah, nigga". Allowing someone to know that in fact, they have it all wrong. While its meaning shifts based on the context, it is generally used as a retort following an incorrect statement, especially one made in jest.
Arthur: Man, what happened to rap after The Chronic? All downhill.
Lester: NNN, it's all good, Dre's still got it!
Rupert: Yo Leroy why was you actin like some kinda jive turkey at da club?
Leroy: NNN dat shit was on pointe. Bes respek.