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Context: The identification and characterisation of populations of youngmassive stars in (giant) HII regions provides important constraints on i) theformation process of massive stars and their early feedback on the environment,and ii) the initial conditions for population synthesis models predicting theevolution of ensembles of stars. Aims: We identify and characterise the stellarpopulations of the following young giant HII regions: M8, G333.6-0.2, andNGC6357. Methods: We acquired H- and K-band spectra of around 200 stars usingThe K-band KMOS on the ESO Very Large Telescope. The targets for M8 and NGC6357were selected from the MYStIX project, which combines X-ray observations withnear-infrared and mid-infrared data. For G333.6-0.2, the sample selection isbased on the near-infrared colours combined with X-ray data. We introduce anautomatic spectral classification method in order to obtain temperatures andluminosities for the observed stars. We analyse the stellar populations usingtheir photometric, astrometric, and spectroscopic properties and compared theposition of the stars in the Hertzprung-Russell diagram with stellar evolutionmodels to constrain their ages and mass ranges. Results: We confirm thepresence of candidate ionising sources in the three regions and report newones, 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 membersof the HII region, whereas a selection based on near infrared (NIR) coloursleads to a membership probability of only $\sim$70\%.

The young stellar content of the giant HII regions M8, G333.6-0.2, and NGC6357 with VLT/KMOS