  We analyse the chemical properties of three z~8 galaxies behind the galaxy
cluster SMACS J0723.3-7327, observed as part of the Early Release Observations
programme of the James Webb Space Telescope (JWST). Exploiting [O III]4363
auroral line detections in NIRSpec spectra, we robustly apply the direct Te
method for the very first time at such high redshift, measuring metallicities
ranging from extremely metal poor (12+log(O/H)~7) to about one-third solar. We
also discuss the excitation properties of these sources, and compare them with
local strong-line metallicity calibrations. We find that none of the considered
diagnostics match simultaneously the observed relations between metallicity and
strong-line ratios for the three sources, implying that a proper re-assessment
of the calibrations may be needed at these redshifts. On the mass-metallicity
plane, the two galaxies at z~7.6 (log(M*/M_sun) = 8.1, 8.7) have metallicities
that are consistent with the extrapolation of the mass-metallicity relation at
z~2-3, while the least massive galaxy at z~8.5 (log(M*/M_sun) = 7.8) shows
instead a significantly lower metallicity . The three galaxies show different
level of offset relative to the Fundamental Metallicity Relation, with two of
them (at z~7.6) being marginally consistent, while the z~8.5 source deviating
significantly, being probably far from the smooth equilibrium between gas
flows, star formation and metal enrichment in place at later epochs.
