details and references.\
We focused only on RRLyrae stars, neglecting all the other different variable types. We first roto-traslated the catalogues of variables to our reference frames by using CataXcorr, a software developed at the Bologna Observatory (Montegriffo, private communication). We then looked for stars in common with our photometry. All the known RRLyrae stars in our FOVs have been recovered in our catalogues. We found 59 RRLyrae in M3, 7 in M13 and 5 in M79 (see Table 1). Their positions in the optical CMDs are shown in Figure \[cmdopt\].

Metallicity and relative ages
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An accurate determination of metallicities and ages is a crucial point for HB morphology studies and for the correct interpretation of comparative analyses.\
The three GGCs in our sample have a similar iron content: according to the recent reassessment of the GC metallicity scale by Carretta et al (2009), metallicities are equal to \[Fe/H\]=$-1.50 \pm 0.05$, $-1.58 \pm 0.04$ and $-1.58 \pm 0.02$ for M3, M13 and M79, respectively. Consistent results were also found by Sneden at al. (2004) who obtained \[Fe/H\]=$-1.55\pm0.02$ and $-1.57\pm0.07$ for M3 and M13 (for homogeneity we report only values obtained from the Lick Hamilton spectra). Only in the analysis 