Methylates precorrin-2 at the C-20 position to produce precorrin-3A. precorrin-2 + S-adenosyl-L-methionine = H(+) + precorrin-3A + S-adenosyl-L-homocysteine precorrin-3B + S-adenosyl-L-methionine = 3 H(+) + precorrin-4 + S-adenosyl-L-homocysteine Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from precorrin-2 (aerobic route): step 1/10. Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from precorrin-2 (aerobic route): step 3/10. Belongs to the precorrin methyltransferase family.