  Context: The identification and characterisation of populations of young
massive stars in (giant) HII regions provides important constraints on i) the
formation process of massive stars and their early feedback on the environment,
and ii) the initial conditions for population synthesis models predicting the
evolution of ensembles of stars. Aims: We identify and characterise the stellar
populations of the following young giant HII regions: M8, G333.6-0.2, and
NGC6357. Methods: We acquired H- and K-band spectra of around 200 stars using
The K-band KMOS on the ESO Very Large Telescope. The targets for M8 and NGC6357
were selected from the MYStIX project, which combines X-ray observations with
near-infrared and mid-infrared data. For G333.6-0.2, the sample selection is
based on the near-infrared colours combined with X-ray data. We introduce an
automatic spectral classification method in order to obtain temperatures and
luminosities for the observed stars. We analyse the stellar populations using
their photometric, astrometric, and spectroscopic properties and compared the
position of the stars in the Hertzprung-Russell diagram with stellar evolution
models to constrain their ages and mass ranges. Results: We confirm the
presence of candidate ionising sources in the three regions and report new
ones, including the first spectroscopically identified O stars in G333.6-0.2.
In M8 and NGC6357, two populations are identified: i) OB main-sequence stars
($M > 5~\rm{M_{\odot}}$) and ii) pre-main sequence stars
($M\approx0.5-5~\rm{M_{\odot}}$). The ages of the clusters are $\sim$1-3~Myr,
$< 3$~Myr, and $\sim$0.5-3~Myr for M8, G333.6-0.2, and NGC6357, respectively.
We show that MYStIX selected targets have $>$ 90\% probability of being members
of the HII region, whereas a selection based on near infrared (NIR) colours
leads to a membership probability of only $\sim$70\%.
