Catalyzes the conversion of alpha-L-mycarosylerythronolide B into erythromycin D in the erythromycin biosynthesis pathway. 3-O-alpha-L-mycarosylerythronolide B + dTDP-alpha-D-desosamine = dTDP + erythromycin D + H(+) kcat is 1 min(-1). Antibiotic biosynthesis; erythromycin biosynthesis. Heterotetramer composed of EryCII and EryCIII. Belongs to the glycosyltransferase 28 family.