  Accurate and precise measurements of masses of galaxy clusters are key to
derive robust constraints on cosmological parameters. Rising evidence from
observations, however, confirms that X-ray masses, obtained under the
assumption of hydrostatic equilibrium, might be underestimated, as previously
predicted by cosmological simulations.
  We analyse more than 300 simulated massive clusters, from `The Three Hundred
Project', and investigate the connection between mass bias and several
diagnostics extracted from synthetic X-ray images of these simulated clusters.
  We find that the azimuthal scatter measured in 12 sectors of the X-ray flux
maps is a statistically significant indication of the presence of an intrinsic
(i.e. 3D) clumpy gas distribution. We verify that a robust correction to the
hydrostatic mass bias can be inferred when estimates of the gas inhomogeneity
from X-ray maps (such as the azimuthal scatter or the gas ellipticity) are
combined with the asymptotic external slope of the gas density or pressure
profiles, which can be respectively derived from X-ray and millimetric
(Sunyaev-Zeldovich effect) observations.
  We also obtain that mass measurements based on either gas density and
temperature or gas density and pressure result in similar distributions of the
mass bias. In both cases, we provide corrections that help reduce both the
dispersion and skewness of the mass bias distribution. These are effective even
when irregular clusters are included leading to interesting implications for
the modelling and correction of hydrostatic mass bias in cosmological analyses
of current and future X-ray and SZ cluster surveys.
