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This paper is the third in our series of papers devoted to the investigation of X-ray emission from OB stars.In our two previous papers,we study the high-resolution X-ray spectra of 32 O stars and 25 B stars to investigate the correlations between the properties of X-ray emission and stellar parameters.We checked if the X-ray hardness and post-shock plasma temperature grow with increasing stellar magnetic field,mass loss rate and terminal wind velocity.Our previous analysis of high-resolution spectra showed that the cor-relations are weak or even absent.In the present paper,we analyzed low-resolution X-ray spectra,using model-independent X-ray hardness values for checking the above mentioned dependencies.We establish that X-ray luminosities Lx weakly depend on the stellar magnetic field.At the same time,Lx oc M0 5 and Lx oc E0.5kin,where M is the mass loss rate and Ekin is the kinetic energy of the wind.The X-ray lumi-nosities decrease with growing magnetic confinement parameter r.We also argue that there is an additional(probably non-thermal)component contributed to the stellar X-ray emission.