Key oxidase enzyme in the biosynthesis of gibberellin. Catalyzes the formation of bioactive gibberellins (GAs) via a three-step oxidation at C-20 of the GA skeleton. Controls the elongation of the vegetative shoot and plant height by the regulation of active gibberellin levels. 2 2-oxoglutarate + gibberellin A12 + H(+) + 3 O2 = 3 CO2 + gibberellin A9 + 2 H2O + 2 succinate 2 2-oxoglutarate + gibberellin A53 + H(+) + 3 O2 = 3 CO2 + gibberellin A20 + 2 H2O + 2 succinate Binds 1 Fe(2+) ion per subunit. Plants silencing GA20OX3 exhibit a semi-dwarf phenotype, and show enhanced resistance against the fungal pathogen Magnaporthe oryzae and the bacterial pathogen Xanthomonas oryzae pv. oryzae, and increased expression of defense-related genes. Plants over-expressing GA20OX3 exhibit a elongated shoot and internode phenotype, and show enhanced susceptibility against the fungal pathogen Magnaporthe oryzae and the bacterial pathogen Xanthomonas oryzae pv. oryzae, and accumulation of the bioactive gibberellins GA1 and GA4. Belongs to the iron/ascorbate-dependent oxidoreductase family.