Histone methyltransferase that specifically trimethylates histone H3 to form H3K9me3. H3K9me3 marks chromatin regions for DNA methylation. {ECO:0000269|PubMed:11713521; ECO:0000269|PubMed:12372305; ECO:0000269|PubMed:12679815; ECO:0000269|PubMed:12887903}.