AMR of the SMC clusters. Left panel: Average metallicity as a function of age (Glatt et al. 2009) for the three SMC GCs analysed in this work (same colour code as Fig. 1). SMC GCs from previous low-resolution works (Parisi et al. 2009, 2015, 2022; Dias et al. 2021) are plotted as open circles. Theoretical AMRs calculated by Pagel & Tautvaisiene (1998) are also reported (the solid curve is the bursting model and the dashed line the continuous model). Right panel: Metallicity distribution of the SMC field stars (Paper I).