No, a 3-point stance is not required for offensive linemen. The NFL rules (7.4.2) cover how a lineman is allowed to switch between a 2-point stance and a 3-point stance before the play (but not the reverse, as that would be a false start).
Item 1. Interior Lineman. It is a False Start if an interior lineman (tackle to tackle) takes or simulates a three-point stance, and then changes his position or moves the hand that is on the ground.
Item 2. Change of Stance. An interior lineman who is in a two-point stance is permitted to reset in a three-point stance or change his position, provided that he resets prior to the snap.
The 2015 NCAA football rulebook does not mention stances at all.
But standing straight up would mean terrible leverage for blocking, and crouching for any length of time without flinching is difficult. So most of the time, linemen use a 3-point stance unless they are playing in a hurry-up offense and the ball is snapped quickly.
Defensive linemen are not required to be in any stance, or even to stop moving. They can do pretty much whatever they want as long as they don't cause the offense to false start.