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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.