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 ($Mapprox0.5-5~rm{M_{odot}}$). 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%.
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