A probably minor shell protein component of the carboxysome, a polyhedral inclusion where RuBisCO (ribulose bisphosphate carboxylase, rbcL-rbcS) is sequestered. This subunit probably does not form homohexamers. Forms mixed heterohexamers with CcmK4, probably with 1:5 CcmK3:CcmK4 stoichiometry. Only very weak interactions with CcmK1 and CcmK2 were seen (PubMed:31603944). Bulky residues in the pore region probably preclude the formation of homohexamers by this subunit (Probable). This cyanobacterium makes beta-type carboxysomes. Belongs to the bacterial microcompartments protein family. CcmK subfamily.