Context : The identification and characterisation of populations of young massive stars in ( giant ) H ii 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 H ii regions : M8 , G333.6-0.2 , and NGC 6357 .

Methods : We have acquired H - and K -band spectra of around 200 stars using The K-band Multi Object Spectrograph ( KMOS ) on the ESO Very Large Telescope .
The targets for M8 and NGC 6357 were selected from the Massive Young Star-Forming Complex Study in Infrared and X-ray ( MYStIX ) , 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 analysed 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 NGC 6357 , two populations are identified : ( i ) OB main-sequence stars ( M > 5 M _ { \odot } ) and ( ii ) pre-main sequence stars ( M \approx 0.5 - 5 ~ { } { 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 NGC 6357 , respectively .
We show that MYStIX selected targets have > 90 % probability of being members of the H ii region , whereas a selection based on near infrared ( NIR ) colours leads to a membership probability of only \sim 70 % .

Conclusions :