Probable L,D-transpeptidase that may perform as-yet-unknown cross-linking reactions in M.tuberculosis. Is not able to generate 3->3 cross-links in peptidoglycan, using tetrapeptide stems as acyl donor substrates. May function in the anchoring of proteins to peptidoglycan. Is irreversibly inactivated by the beta-lactams carbapenems via the formation of a covalent adduct resulting from enzyme acylation; imipenem is the most efficient drug for in vitro LdtMt3 inactivation. Cell wall biogenesis; peptidoglycan biosynthesis.