Catalysis of the reaction: S-adenosyl-L-methionine + (histone H3)-arginine (position 26) = S-adenosyl-L-homocysteine + (histone H3)-N-methyl-arginine (position 26). This reaction is the addition of a methyl group to the arginine residue at position 26 of histone H3.
histone-H3R26 methyltransferase activity