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The study of the "chromosome maps" of Galactic Globular Clusters has shownthat the stars identified as `first generation' often define an extendedsequence in the $m_{F275W}-m_{F814W}$ colour, whose straightforwardinterpretation, by comparison with synthetic spectra, is that they areinhomogeneous in helium content.  The cluster M 3 (NGC 5272) is one of the most prominent example of thisphenomenon, since its first generation is distributed on an extended colourrange, formally corresponding to a large helium enhancement ($ \sim 0.1$). Itis necessary to ask whether the bulk of photometric observations available forthis cluster supports or falsifies this interpretation. For this purpose, weexamine the horizontal branch morphology, the period and magnitudedistributions of the RR Lyrae variables, and the main sequence colourdistribution. Simulating the first generation stars with such internalvariation of helium content we can not meet all the observational constraintsat the same time, concluding that the origin of the first generation colourspread is still without a straightforward explanation.

Is helium the key parameter in the extended color spread of the first generation stars in M3?