  The inner structure of core-helium burning (CHeB) stars remains uncertain due
to the yet unknown nature of mixing at the boundary of their cores. Large
convective cores beyond a bare Schwarzschild model are favoured both from
theoretical arguments and from asteroseismological constraints. However, the
exact nature of this extra mixing, and in particular the possible presence of
semiconvective layers, is still debated. In this work, we approach this problem
through a new avenue by performing the first full-sphere 3D hydrodynamics
simulations of the interiors of CHeB stars. We use the PPMstar explicit gas
dynamics code to simulate the inner 0.45 $M_{\odot}$ of a 3 $M_{\odot}$ CHeB
star. Simulations are performed using different Cartesian grid resolutions
(768$^3$, 1152$^3$ and 1728$^3$) and heating rates. We use two different
initial states, one based on MESA's predictive mixing scheme (which
significantly extends the core beyond the Schwarzschild boundary) and one based
on the convective premixing approach (which exhibits a semiconvective
interface). The general behaviour of the flow in the convective core and in the
stable envelope (where internal gravity waves are observed) is consistent with
our recent simulations of core convection in massive main-sequence stars, and
so are the various luminosity scaling relations. The semiconvective layers are
dominated by strong internal gravity waves that do not produce measurable
species mixing, but overshooting motions from the convective core gradually
homogenize the semiconvective interface. This process can possibly completely
erase the semiconvective layers, which would imply that CHeB stars do not
harbour a semiconvection zone.
