  The study of the "chromosome maps" of Galactic Globular Clusters has shown
that the stars identified as `first generation' often define an extended
sequence in the $m_{F275W}-m_{F814W}$ colour, whose straightforward
interpretation, by comparison with synthetic spectra, is that they are
inhomogeneous in helium content.
  The cluster M 3 (NGC 5272) is one of the most prominent example of this
phenomenon, since its first generation is distributed on an extended colour
range, formally corresponding to a large helium enhancement ($ \sim 0.1$). It
is necessary to ask whether the bulk of photometric observations available for
this cluster supports or falsifies this interpretation. For this purpose, we
examine the horizontal branch morphology, the period and magnitude
distributions of the RR Lyrae variables, and the main sequence colour
distribution. Simulating the first generation stars with such internal
variation of helium content we can not meet all the observational constraints
at the same time, concluding that the origin of the first generation colour
spread is still without a straightforward explanation.
