inferred from their MS or spectroscopic analyses (NGC2808, $\omega$ Centauri – Bedin et al. 2004, NGC6752 – Milone et al. 2010, NGC6397 – Milone et al. 2012; NGC1851 – Gratton et al. 2012)\
In order to further investigate the *[“i-th parameter problem”]{}*, we selected a triplet of GGCs with similar metallicity ($[Fe/H]\sim-1.50$) and age, namely NGC5272 (M3), NGC6205 (M13) and NGC1904 (M79) as templates for a comparative analysis of their HBs. These targets are basically the same used by Ferraro et al. (1997, 1998), with the only exception of M80, which have been replaced by M79 since recent spectroscopic analyses (Carretta et al. 2009) show that it is slightly more metal-poor than the others. As shown by Ferraro et al. (1998), M13 has a very extended HB with two clear gaps (named G1 and G2 after their analysis); while, the HB of M3 covers a much narrower extension in effective temperature and shows only a mild indication for a gap at the same $T_{\rm eff}$ as G1 in M13. M79 represents an intermediate case between the two.\
As already done for NGC2808 (Paper I) we will use a combination of optical and UV photometric images. Details about data reduction are 