  We present the first large-scale 3D kinematic study of ~2000
spectroscopically-confirmed young stars (<20 Myr) in 18 star clusters and OB
associations (hereafter groups) from the combination of Gaia astrometry and
Gaia-ESO Survey spectroscopy. We measure 3D velocity dispersions for all
groups, which range from 0.61 to 7.4 km/s (1D velocity dispersions of 0.35 to
4.3 km/s). We find the majority of groups have anisotropic velocity
dispersions, suggesting they are not dynamically relaxed. From the 3D velocity
dispersions, measured radii and estimates of total mass we estimate the virial
state and find that all systems are super-virial when only the stellar mass is
considered, but that some systems are sub-virial when the mass of the molecular
cloud is taken into account. We observe an approximately linear correlation
between the 3D velocity dispersion and the group mass, which would imply that
the virial state of groups scales as the square root of the group mass.
However, we do not observe a strong correlation between virial state and group
mass. In agreement with their virial state we find that nearly all of the
groups studied are in the process of expanding and that the expansion is
anisotropic, implying that groups were not spherical prior to expansion. One
group, Rho Oph, is found to be contracting and in a sub-virial state (when the
mass of the surrounding molecular cloud is considered). This work provides a
glimpse of the potential of the combination of Gaia and data from the next
generation of spectroscopic surveys.
