  Up-to-date isochrones, zero-age horizontal branch (ZAHB) loci, and
evolutionary tracks for core He-burning stars are applied to the
color-magnitude diagrams (CMDs) of M3, M15, and M92, focusing in particular on
their RR Lyrae populations. Periods for ab- and c-type variables are calculated
using the latest theoretical calibrations of log P_ab and log P_c as a function
of luminosity, mass, effective temperature, and metallicity. Our models are
generally able to reproduce the measured periods to well within the
uncertainties implied by the stellar properties on which pulsation periods
depend, as well as the mean periods and cluster-to-cluster differences in
<P_ab> and <P_c>, on the assumption of well-supported values of E(B-V),
(m-M)_V, and [Fe/H]. While many of the RR Lyrae in M3 lie close to the same
ZAHB that fits the faintest HB stars at bluer or redder colors, the M92
variables are all significantly evolved stars from ZAHB locations on the blue
side of the instability strip. M15 appears to contain a similar population of
HB stars as M92, along with additional helium-enhanced populations not present
in the latter which comprise most of its RR Lyrae stars. The large number of
variables in M15 and the similarity of the observed values of <P_ab> and <P_c>
in M15 and M92 can be explained by HB models that allow for variations in Y.
Similar ages (~12.5 Gyr) are found for all three clusters, making them
significantly younger than the field halo subgiant HD 140283. Our analysis
suggests a preference for stellar models that take diffusive processes into
account.
