Catalyzes the addition of molecular CO(2) and H(2)O to ribulose 1,5-bisphosphate (RuBP), generating two molecules of 3-phosphoglycerate (3-PGA). Functions in an archaeal AMP degradation pathway, together with AMP phosphorylase and R15P isomerase. 2 (2R)-3-phosphoglycerate + 2 H(+) = CO2 + D-ribulose 1,5-bisphosphate + H2O D-ribulose 1,5-bisphosphate + O2 = (2R)-3-phosphoglycerate + 2-phosphoglycolate + 2 H(+) Binds 1 Mg(2+) ion per subunit. Reversibly inhibited by O(2). Active over a broad temperature range. Homodimer. In contrast to form I RuBisCO, the form III RuBisCO is composed solely of large subunits. Because the Archaea possessing a type III RuBisCO are all anaerobic, it is most likely that only the carboxylase activity of RuBisCO, and not the competitive oxygenase activity (by which RuBP reacts with O(2) to form one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate), is biologically relevant in these strains. Belongs to the RuBisCO large chain family. Type III subfamily.