Catalysis of the reaction: S-adenosyl-L-methionine + (histone H2A)-arginine (position 3) = S-adenosyl-L-homocysteine + (histone H2A)-N-methyl-arginine (position 3). This reaction is the addition of a methyl group to the arginine residue at position 3 of histone H2A.
histone-H2AR3 methyltransferase activity