  Non-spherical structure in massive stars at the point of iron core collapse
can have a qualitative impact on the properties of the ensuing core-collapse
supernova explosions and the multi-messenger signals they produce. Strong
perturbations can aid successful explosions by strengthening turbulence in the
post-shock region. Here, we report on a set of $4\pi$ 3D hydrodynamic
simulations of O- and Si-shell burning in massive star models of varied initial
masses using MESA and the FLASH simulation framework. We evolve four separate
3D models for roughly the final ten minutes prior to, and including, iron core
collapse. We consider initial 1D MESA models with masses of 14-, 20-, and 25
$M_{\odot}$ to survey a range of O/Si shell density and compositional
configurations. We characterize the convective shells in our 3D models and
compare them to the corresponding 1D models. In general, we find that the
angle-average convective speeds in our 3D simulations near collapse are three
to four times larger than the convective speeds predicted by MESA at the same
epoch for our chosen mixing length parameter of $\alpha_{\rm{MLT}}=1.5$. In
three of our simulations, we observe significant power in the spherical
harmonic decomposition of the radial velocity field at harmonic indices of
$\ell=1-3$ near collapse. Our results suggest that large-scale modes are common
in massive stars near collapse and should be considered a key aspect of
pre-supernova progenitor models.
