Methyltransferase which catalyzes the formation of N(2)-methylguanosine at position 6 in a broad range of tRNA substrates containing the characteristic 3'-CCA terminus of mature tRNAs (By similarity). Also catalyzes the formation of N(2)-methylguanosine at position 7 of tRNA(Trp) (By similarity). Requires the methyltransferase adapter protein TRM112 for tRNA methyltransferase activity (By similarity). guanosine(6) in tRNA + S-adenosyl-L-methionine = H(+) + N(2)-methylguanosine(6) in tRNA + S-adenosyl-L-homocysteine Interacts with TRMT112; the interaction is direct and is required for THUMPD3 methyltransferase activity. Ubiquitously expressed (PubMed:9108056). Abundantly expressed in the testis, also expressed in the brain, heart, kidney, liver, lung, muscle and spleen (PubMed:34669960). Belongs to the methyltransferase superfamily.