In Diablo 3 for the PC, you can’t just jump into the hardest difficulties right away, because the game is level and equipment based. You might think you were a D2 badass, but trust me.. the hardest difficulty in D3 is not done lightly.
In order to unlock the different difficulties, you just need to beat them, and attain a certain level.
Nightmare Difficulty – Beat all four Acts on Normal, and be level 30
Hell Difficulty - Beat all four Acts on Nightmare, and be level 50
Inferno Difficulty - Beat all four Acts on Hell, and be level 60