  The large number of variables in M3 provides a unique opportunity to study an
extensive sample of variables with the same apparent distance modulus. Recent,
high accuracy CCD time series of the variables show that according to their
mean magnitudes and light curve shapes, the variables belong to four separate
groups. Comparing the properties of these groups (magnitudes and periods) with
horizontal branch evolutionary models, we conclude that these samples can be
unambiguously identified with different stages of the horizontal branch stellar
evolution. Stars close to the zero age horizontal branch (ZAHB) show Oosterhoff
I type properties, while the brightest stars have Oosterhoff II type statistics
regarding their mean periods and RRab/RRc number ratios. This finding
strengthens the earlier suggestion of Lee et al. (1990) connecting the
Oosterhoff dichotomy to evolutionary effects, however, it is unexpected to find
large samples of both of the Oosterhoff type within a single cluster, which is,
moreover, the prototype of the Oosterhoff I class globular clusters. The very
slight difference between the Fourier parameters of the stars (at a given
period) in the three fainter samples spanning over about 0.15 mag range in M_V
points to the limitations of any empirical methods which aim to determine
accurate absolute magnitudes of RR Lyrae stars solely from the Fourier
parameters of the light curves.
