Reversible hydration of carbon dioxide. H(+) + hydrogencarbonate = CO2 + H2O Activated by proton donors such as imidazole and the dipeptide histidylhistidine (PubMed:16042381). Inhibited by coumarins and sulfonamide derivatives such as acetazolamide (PubMed:18618712, PubMed:19186056, PubMed:19206230). Muscle specific. At 6 weeks gestation, transcripts accumulate at low levels in the somites and at high levels throughout the notochord. As gestation continues, CA3 becomes abundant in all developing muscle masses and continues at high to moderate levels in the notochord. S-thiolated both by thiol-disulfide exchange with glutathione disulfide and by oxyradical-initiated S-thiolation with reduced glutathione. S-glutathionylated in hepatocytes under oxidative stress. Belongs to the alpha-carbonic anhydrase family.