S-adenosyl-L-methionine-dependent methyltransferase that mediates N(3)-methylcytidine modification of residue 32 of the tRNA anticodon loop of tRNA(Ser), including tRNA(Ser)(UGA) and tRNA(Ser)(GCU). Interaction with SARS1/SerRS is required for N(3)-methylcytidine methylation. cytidine(32) in tRNA(Ser) + S-adenosyl-L-methionine = H(+) + N(3)-methylcytidine(32) in tRNA(Ser) + S-adenosyl-L-homocysteine Monomer. Interacts with SARS1/SerRS; interaction is mediated via tRNA(Ser) and is required for N(3)-methylcytidine methylation. Belongs to the methyltransferase superfamily. METL family.