We present the results of long-slit spectroscopy in several slit positions that cover different morphological structures of the central parts of three bright Galactic HII regions: M8, M17 and NGC 7635. We study the spatial distributions of a large number of nebular parameters, such as the extinction coefficient, line fluxes, physical conditions and ionic abundances at the maximum spatial resolution attainable with our instrumentation. Particularly, our goal is to study the behaviour of the abundance discrepancy factor of O2+ [ADF(O2+)], defined as the logarithmic difference of the O2+ abundances derived from collisionally excited and recombination lines. We find that ADF(O2+) remains fairly constant along the slit positions of M8 and M17. For NGC 7635, we only detect the OII recombination lines in the integrated spectrum along the whole slit, where ADF(O2+) reaches a remarkably high value of about 0.59 dex. We compare our results with previous results obtained for the Orion Nebula. We find much evidence to suggest the presence of a candidate Herbig-Haro object in M8. Based on observations made with the 4.2-m William Herschel Telescope (WHT) operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. E-mail: amd [at] iac.es (amd[at]iac[dot]es)