We analyse the chemical properties of three z similar to 8 galaxies behind the galaxy cluster SMACSJ0723.3-7327, observed as part of the Early Release Observations programme of the James Webb Space Telescope. Exploiting [O III]lambda 4363 auroral line detections in NIRSpec spectra, we robustly apply the direct T-e method for the very first time at such high redshift, measuring metallicities ranging from extremely metal poor (12 + log(O/H)approximate to 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 similar to 7.6 (log(M-*/M-circle dot) = 8.1, 8.7) have metallicities that are consistent with the extrapolation of the mass-metallicity relation at z similar to 2-3, while the least massive galaxy at z similar to 8.5 (Log(M-*/M-circle dot) = 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 similar to 7.6) being marginally consistent, while the z similar to 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.