In the framework of our narrow-band survey of Local Group galaxies, we present the results of the search for planetary nebulae (PNe) in three spheroidal companions to the Andromeda galaxy. We find 9 candidate PNe in NGC 147, 5 in NGC 185, and 75 in the ˜0.4 square degree area searched around NGC 205, increasing the number of PNe known in these galaxies. It is shown that in the crowded regions of these galaxies continuum-subtracted images are more effective in detecting PNe than colour-colour diagrams obtained via automatic photometry. For NGC 205, the degree of contamination of PNe belonging to the halo of M 31 is estimated; taking it into account, 35 PNe within 1.5 tidal radii from the centre of NGC 205 have been used to build its PN luminosity function. Candidate PNe in NGC 185 are systematically brighter than those in NGC 147. Considering that star formation is thought to have been much stronger in NGC 185 than in NGC 147 in the last 3 Gyr, this might suggest that the bright end of the PN luminosity function is populated by relatively massive stars, as predicted by some recent theoretical models. This result, however, has to be taken with some caution, given the small PN population size of these galaxies and a rather incomplete knowledge of their star formation history. Based on observations obtained at the 2.5m INT telescope 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.