An analysis is made of the chemical composition of planetary nebulae (PN) in three different systems: the Galaxy, comprising disk and bulge objects, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The data has been gathered by the IAG/USP group along the last decade in a homogeneous procedure, involving observational techniques, data acquisition and reduction. Emphasis is placed on distance-independent correlations, which are particularly important for PN in the galactic disk, for which individual distances are poorly known. It is shown that most abundance correlations hold for all three systems, taking into account the observed metallicity differences between the Galaxy and the Magellanic Clouds.