Catalyzes the oxidative phosphorylation of glyceraldehyde 3-phosphate (G3P) to 1,3-bisphosphoglycerate (BPG) using the cofactor NAD. The first reaction step involves the formation of a hemiacetal intermediate between G3P and a cysteine residue, and this hemiacetal intermediate is then oxidized to a thioester, with concomitant reduction of NAD to NADH. The reduced NADH is then exchanged with the second NAD, and the thioester is attacked by a nucleophilic inorganic phosphate to produce BPG. D-glyceraldehyde 3-phosphate + NAD(+) + phosphate = (2R)-3-phospho-glyceroyl phosphate + H(+) + NADH Carbohydrate degradation; glycolysis; pyruvate from D-glyceraldehyde 3-phosphate: step 1/5. Carbohydrate biosynthesis; gluconeogenesis. Homotetramer. Expressed in vegetative cells and heterocysts, although expression is substantial in both cell types, it is higher in heterocysts. Gap3 is certainly the main GAPDH present in cells, since no significant Gap1 is detected in cells under different growth conditions. Belongs to the glyceraldehyde-3-phosphate dehydrogenase family.