Metallicity distribution of the Milky Way (868 stars), LMC (1561 stars), and SMC (674 stars) from empirical metallicity relations in the I band: (a) small points represent stars with metallicity from the literature while bigger points are new metallicity estimation. Dashed circle encloses the solar neighbour region which concentrates most of the spectroscopic measurements; (b), (d), and (f): Estimated Cepheid metallicities versus the galactocentric distance for MW, LMC, and SMC. Red dashed lines are the 1σ deviation in each bin of 1 kpc around the median value of each bin. MW and LMC maps are plotted in a larger size in Figs. C.1 and C.2. (a) MW on-view. (c) LMC. (e) SMC.