  We present the radial gas-phase, mass-weighted metallicity profiles and
gradients of the TNG50 star-forming galaxy population measured at redshifts
$z=$ 0--3. We investigate the redshift evolution of gradients and examine
relations between gradient steepness and galaxy properties. We find that TNG50
gradients are predominantly negative at all redshifts, although we observe
significant diversity among these negative gradients. We determine that the
gradient steepness of all galaxies increases approximately monotonically with
redshift at a roughly constant rate. This rate does not vary significantly with
galaxy mass. We observe a weak negative correlation between gradient steepness
and galaxy stellar mass at redshifts $z\leq2$. However, when we normalize
gradients by a characteristic radius defined by the galactic star formation
distribution, we find that these normalized gradients remain invariant with
both stellar mass and redshift. We place our results in the context of previous
simulations and show that TNG50 high-redshift gradients are steeper than those
of models featuring burstier feedback, which may further highlight
high-redshift gradients as important discriminators of galaxy formation models.
We also find that redshift $z=0$ and $z=0.5$ TNG50 gradients are consistent
with the gradients observed in galaxies at these redshifts, although the
preference for flat gradients observed in redshift $z\gtrsim1$ galaxies is not
present in TNG50. If future JWST and ELT observations validate these flat
gradients, it may indicate a need for simulation models to implement more
powerful radial gas mixing within the ISM, possibly via turbulence and/or
stronger winds
