proper roto-traslations. In particular, for each cluster the instrumental coordinates have been reported to the GSCII astrometric standard system by using a large number of primary and secondary standards in common, consisting typically of Wide Field ground-based data-sets, in the WFPC2 Fields of View (FOVs) (see Lanzoni et al. 2007 for details). At the end of the procedure, the typical error of the astrometric solution is $\sim 0.2\arcsec$.

RR Lyrae identification
-----------------------

The identification of variable stars lying along the HB is a crucial step to perform a suitable comparison between observations and theoretical models. This is particularly true in case of M3, that hosts tens of RR Lyrae stars.\
However, none of the data-sets used here is designed for variability search, both because of the short time baseline covered and number of images acquired. In our CMDs, variables are basically sampled at random phase. Thus we decided to use publicly available catalogues of variable stars in order to cross-identify them in our photometric catalogues, as already done in Paper I. We retrieved position, magnitude, and classification lists from the [“Catalogue of Variable Stars in Galactic Globular Clusters (2011)”]{}[^4] (Clement et al. 2001). We refer to that paper for all 