  We present Str\"omgren photometry of the Galactic Globular Cluster M3 to
study its multiple generations phenomenon. The use of different
colour-magnitude diagrams and especially of the notoriously efficient c_y index
allowed us to detect a double Red Giant Branch in the cluster CMD. After
decontamination from fore- and background sources, the two sequences turned out
to be equally populated. The two components also show a bimodal radial
distribution well corresponding to that predicted by numerical simulations for
clusters living in an intermediate dynamical evolutive state and with a
population with modified chemical composition that was born more centrally
concentrated than the primordial. The analysis of high-resolution spectra
quantitatively demonstrates that the two detected sequences correspond to the
first (Na-poor) generation and the second (Na-rich) generation, thus confirming
the importance of synergy between photometry and spectroscopy.
