Running back gets in a three point stance, after everyone is set and during QB cadence running back gets out three point stance and goes in motion. Is that legal or considered a false start.
YOUR judgment call. Vast majority of the time, the back's initial step will be forward, before going in motion either way. As long as that initial step is appropriate, not exaggerated or artificially explosive AND there is no instant reaction by the defense (that YOU judge to be artificially motivated), and the action by the back is consistent with similar behavior in similar previous situations there's likely no foul either way (when the defender didn't violate the NZ)
However, (Often coincidentally to critical short yardage situations, or when the back suddenly and dramatically explodes out of his set position, for that 1st step, before going in motion either way, judging the initial movement to be deliberate and potentially intended to draw a defender into the NZ,
judging the action as a false start (whether or NOT it successfully drew the defense into the NZ, is an appropriate call). Even when the defense does NOT react, recognizing the intent with a flag, is very likely to eliminate repeated attempts during the contest (by either team).
When that initial movement by the back, does draw the defense into violating the NZ, YOU have to judge whether the violation was caused by the defensive player's anticipation, or the movement of the offensive back, that "drew", the defensive player
(whether intentionally, or NOT) into the NZ