  Multidimensional hydrodynamic simulations of shell convection in massive
stars suggest the development of aspherical perturbations that may be amplified
during iron core-collapse. These perturbations have a crucial and qualitative
impact on the delayed neutrino-driven core-collapse supernova explosion
mechanism by increasing the total stress behind the stalled shock. In this
paper, we investigate the properties of a 15 \msun model evolved in 1-,2-, and
3-dimensions (3D) for the final $\sim$424 seconds before gravitational
instability and iron core-collapse using MESA and the FLASH simulation
framework. We find that just before collapse, our initially perturbed fully 3D
model reaches angle-averaged convective velocity magnitudes of $\approx$
240-260 km s$^{-1}$ in the Si- and O-shell regions with a Mach number $\approx$
0.06. We find the bulk of the power in the O-shell resides at large scales,
characterized by spherical harmonic orders ($\ell$) of 2-4, while the Si-shell
shows broad spectra on smaller scales of $\ell\approx30-40$. Both convective
regions show an increase in power at $\ell=5$ near collapse. We show that the
1D \texttt{MESA} model agrees with the convective velocity profile and speeds
of the Si-shell when compared to our highest resolution 3D model. However, in
the O-shell region, we find that \texttt{MESA} predicts speeds approximately
\emph{four} times slower than all of our 3D models suggest. All eight of the
multi-dimensional stellar models considered in this work are publicly
available.
