We analyze the PNe chemical behavior in three different galaxies, two dwarf irregulars and one spiral. Different behaviors are found. In the very low metallicity galaxy NGC 3109, PNe analyzed appear 0.39 dex O-richer than HII regions, while Ar/H ratio is, in average, 0.15 dex poorer. We interpret this as an evidence of significant O dredge-up in these LIMS, born in a very low metallicity environment. In NGC 6822, with a present metallicity 12+log O/H=8.06, two PN populations were found. A young one, with abundances similar to those in HII regions and an old population, with metallicities a factor of two lower. In this case no strong evidence for O dredge-up in LIMS is found. Therefore, metallicities lower than 12+log O/H =7.7 are required for an efficient O dredge-up. From our preliminary analysis of the abundances of PNe in NGC 300 we find that they are similar to the abundances in HII regions. Apparently, the PNe analyzed belong to a young population. Very similar abundance gradients, with galactocentric distance, are found for HII regions and for PNe.