  Galaxy clusters grow by accreting galaxies from the field and along filaments
of the cosmic web. As galaxies are accreted they are affected by their local
environment before they enter (pre-processing), and traverse the cluster
potential. Observations that aim to constrain pre-processing are challenging to
interpret because filaments comprise a heterogeneous range of environments
including groups of galaxies embedded within them and backsplash galaxies that
contain a record of their previous passage through the cluster. This motivates
using modern cosmological simulations to dissect the population of galaxies
found in filaments that are feeding clusters, to better understand their
history, and aid the interpretation of observations. We use zoom-in simulations
from The ThreeHundred project to track halos through time and identify their
environment. We establish a benchmark for galaxies in cluster infall regions
that supports the reconstruction of the different modes of pre-processing. We
find that up to 45% of all galaxies fall into clusters via filaments (closer
than 1Mpc/h from the filament spine). 12% of these filament galaxies are
long-established members of groups and between 30 and 60% of filament galaxies
at R200 are backsplash galaxies. This number depends on the cluster's dynamical
state and sharply drops with distance. Backsplash galaxies return to clusters
after deflecting widely from their entry trajectory, especially in relaxed
clusters. They do not have a preferential location with respect to filaments
and cannot collapse to form filaments. The remaining pristine galaxies (30 -
60%) are environmentally effected by cosmic filaments alone.
