  One-dimensional (1D) stellar evolution models are widely used across various
astrophysical fields, however they are still dominated by important
uncertainties that deeply affect their predictive power. Among those, the
merging of independent convective regions is a poorly understood phenomenon
predicted by some 1D models but whose occurrence and impact in real stars
remain very uncertain. Being an intrinsically multi-D phenomenon, it is
challenging to predict the exact behaviour of shell mergers with 1D models. In
this work, we conduct a detailed investigation of a multiple shell merging
event in a 20 M$_\odot$ star using 3D hydrodynamic simulations. Making use of
the active tracers for composition and the nuclear network included in the 3D
model, we study the merging not only from a dynamical standpoint but also
considering its nucleosynthesis and energy generation. Our simulations confirm
the occurrence of the merging also in 3D, but reveal significant differences
from the 1D case. Specifically, we identify entrainment and the erosion of
stable regions as the main mechanisms that drive the merging, we predict much
faster convective velocities compared to the mixing-length-theory velocities,
and observe multiple burning phases within the same merged shell, with
important effects for the chemical composition of the star, which presents a
strongly asymmetric (dipolar) distribution. We expect that these differences
will have important effects on the final structure of massive stars and thus
their final collapse dynamics and possible supernova explosion, subsequently
affecting the resulting nucleosynthesis and remnant.
