the estimated photometric errors we build two synthetic populations of similar size to the observed ones for each comparison. With the typical uncertainties of our HST catalogues at the TO magnitude level, an age difference of $1.5$ Gyr would be clearly visible and detectable. We then conclude that M13, M3 and M79 differ in age by less than $1-1.5$ Gyr. This result is in agreement with estimates by several authors (Johnson & Bolte 1998; Rosenberg et al. 1999; Salaris and Weiss 2002; Marin-Franch et al. 2009; D10), while it is incompatible with the age differences proposed by G10. It is worth noticing that an age difference of about $1.5-2$ Gyr would be compatible with the observed data only assuming extremely different He abundances ($\Delta Y\sim0.1$) between these clusters.

Theoretical framework {#theory}
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We have shown that M3, M13 and M79 have similar metallicities and are coeval within typical uncertainties. Therefore in this case, the scenario proposed by G10 and D10, identifying the [*second parameter*]{} with age, is not applicable. We therefore investigate here the possible role of variations of Helium abundances in the framework of GCs having experienced multiple formation bursts (D’Ercole et al. 2008; Conroy & Spergel 2011; Valcarce 