Catalysis of the reaction: L-histidine + 3 S-adenosyl-L-methionine <=> 3 H(+) + hercynine + 3 S-adenosyl-L-homocysteine. This reaction is the addition of three methyl groups to L-histidine to form N-alpha,N-alpha,N-alpha-trimethyl-L-histidine (also known as hercynine).
histidine-alpha-N-methyltransferase activity