Methylates cobalt-precorrin-2 at the C-20 position to produce cobalt-precorrin-3A in the anaerobic cobalamin biosynthesis pathway. Co-precorrin-2 + S-adenosyl-L-methionine = Co-precorrin-3 + H(+) + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from sirohydrochlorin (anaerobic route): step 2/10. Belongs to the precorrin methyltransferase family.