  As potential progenitors of several exotic phenomena including gravitational
wave sources, magnetic stars, and Be stars, close massive binary systems probe
a crucial area of the parameter space in massive star evolution. Despite the
importance of these systems, large uncertainties regarding the nature and
efficiency of the internal mixing mechanisms still exist. In this work, we aim
to provide robust observational constraints on the internal mixing processes by
spectroscopically analyzing a sample of three massive overcontact binaries at
different metallicities. Using optical phase-resolved spectroscopic data, we
perform an atmosphere analysis using more traditional 1D techniques and using
state-of-the-art 3D techniques. We compare and contrast the assumptions and
results of each technique and investigate how the assumptions affect the final
derived atmospheric parameters. We find that in all three cases, both
components of system are highly overluminous indicating either efficient
internal mixing of helium or previous non-conservative mass transfer. However,
we do not find strong evidence of helium or CNO surface abundance changes
usually associated with mixing. Additionally, we find that in unequal mass
systems, the measured effective temperature and luminosity of the less massive
component places it very close to the more massive component on the
Hertzsprung-Russell diagram. These results were obtained independently using
both of the techniques mentioned above, which suggests that these measurements
are robust. The observed discrepancies between the temperature and the surface
abundance measurements when compared to theoretical expectations indicate that
unaccounted for additional physical mechanisms may be at play.
