Involved in the biosynthesis of the macrolide antibiotic erythromycin. Catalyzes the reversible transfer of mycarosyl from dTDP-beta-L-mycarose to erythronolide B to yield 3-alpha-L-mycarosylerythronolide B. It can also use TDP-beta-L-cladinose. dTDP-beta-L-mycarose + erythronolide B = 3-O-alpha-L-mycarosylerythronolide B + dTDP + H(+) It does not require an auxiliary activator protein. Belongs to the glycosyltransferase 28 family.