Take the over(barely). Even assuming Robert Griffin III makes 16 starts(which is unlikely), the Redskins are going to run the ball far more than they did even in 2014. They ran it about 40% in 2014, which is much more than Gruden likes to run it. He prefers to run it 33% of the time at most.
If the defense struggles badly(again) and the Skins are down in the 2nd halves of most games, then all bets are off. Robert could post over 3400 yards if his offensive line gels, but he could also struggle to 3000 yards if defenses can still get in his face consistently.