Histone methyltransferase that trimethylates histone H3 'Lys-36' forming H3K36me3 (By similarity). Involved in transcription elongation as well as in transcription repression (PubMed:16087749). L-lysyl(36)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(36)-[histone H3] + 3 S-adenosyl-L-homocysteine The AWS and SET domains are necessary for transcription repression. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. SET2 subfamily.