  The stellar metallicity is a direct measure of the amount of metals present
in a galaxy, as a large part of the metals lie in its stars. In this paper we
investigate new stellar metallicity indicators suitable for high-z galaxies
studying the stellar photospheric absorption lines in the rest frame
ultraviolet, hence sampling predominantly young hot stars. We defined these new
indicators based on the equivalent widths (EW) of selected features using
theoretical spectra created with the evolutionary population synthesis code
Starburts99. We used them to compute the stellar metallicity for a sample of
UV-selected galaxies at z > 3 from the AMAZE survey using very deep (37h per
object) VLT/FORS spectra. Moreover, we applied the new metallicity indicators
to eight additional high redshift galaxies found in literature. We then
compared stellar and gas-phase metallicities measured from the emission lines
for all these galaxies, finding that within the errors the two estimates are in
good agreement, with possible tendency to have stellar metallicities lower than
the gas phase ones. For the first time, we are able to study the stellar
mass-metallicity relation at z > 3. We find that the metallicity of young, hot
stars in galaxies at z \sim 3 have similar values of the aged stars in local
SDSS galaxies, contrary to what observed for the gas phase metallicity.
