Disclosed are fused heterotricyclic compounds of the following formula I, or an enantiomer, diastereomer, tautomer or pharmaceutically acceptable salt thereof, wherein one of A ring or C ring is a 5-7 membered unsaturated heterocyclic ring. The disclosed compounds are useful as inhibitors of 17β-hydroxysteroid dehydrogenase 3 (17β-HSD3). Also disclosed are methods of using such compounds in the treatment of hormone sensitive diseases such as prostate cancer, and pharmaceutical compositions comprising such compounds.