presented in Section 2. In Section 3 we will focus on the long-standing debate about the age-differences between these clusters. Comparisons with theoretical models and cluster-to-cluster differences will be discussed in Section 4 and 5.

![Optical CMDs of the three GGCs considered in our analysis.[]{data-label="cmdopt"}](f1.ps){width="84mm"}

Observational data sets and data reduction
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The data-set used in the present work consists of Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) images. All targets have been observed in the $F555W$, $F336W$ and $F160BW$ bands, the only exception is M3 for which no F160BW data are available. (see Table 1). Images and photometry obtained in other bands are also available for some of the clusters, however as shown in Rood et al. 2008 and Paper I the combination of $F555W$, $F336W$ and $F160BW$ seems to be the best filters setup to study both RHBs and stars lying at the very hot end of the HB. The photometric catalogues of M3 and M13 have been already partially presented by Ferraro et al. (1998), while the photometry of M79 has been already used by Lanzoni et al. (2007). Moreover Far-UV photometry of M79 and M13 has been presented by Rood et al. (2008) 