The function of COQ3 (coenzyme Q3, methyltransferase, ENSG00000132423) is as follows. O-methyltransferase required for two non-consecutive steps during ubiquinone biosynthesis (By similarity) (PubMed:10777520, PubMed:38425362). Catalyzes the 2 O-methylation of 3,4-dihydroxy-5- (all-trans-decaprenyl)benzoic acid into 4-hydroxy-3-methoxy-5-(all- trans-decaprenyl)benzoic acid (By similarity) (PubMed:10777520, PubMed:38425362). Also catalyzes the last step of ubiquinone biosynthesis by mediating methylation of 3-demethylubiquinone into ubiquinone (By similarity) (PubMed:38425362). Also able to mediate the methylation of 3-demethylubiquinol-10 into ubiquinol-10 (By similarity) (PubMed:10777520). {ECO:0000255|HAMAP-Rule:MF_03190, ECO:0000269|PubMed:10777520, ECO:0000269|PubMed:38425362}.