त्रिपुरविजयः
त्रि · पुर · विजय
त्रयाणां पुराणां समाहारः
द्विगु — the three cities taken as one thing
त्रिपुरस्य विजयः
तत्पुरुष (genitive) — the conquest of it
तत्पुरुष. the conquest of the Three Cities — Śiva's one great victory, the burning of the three flying fortresses of the demons, which is what a mountain choir would sing about. **Mallinātha spends his whole note on the first member and none of it on the second**, and what he settles there is a gender. His vigraha is त्रयाणां पुराणां समाहारः, 'the taking-together of three cities' — a समाहार, a collective, which is what makes the compound a dvigu rather than a plain numeral phrase. He licenses the compound by Pāṇini 2.1.51 तद्धितार्थोत्तरपदसमाहारे च, citing it by its opening words as the school does. Then पात्रादित्वान्नपुंसकत्वम्, 'its neuter gender comes of its being in the पात्रादि class': a collective compound is neuter to begin with by 2.4.17 स नपुंसकम्, and 4.1.21 द्विगोः would take an a-final dvigu into the feminine, and the पात्रादि list is the set of dvigus that rule is held back from. So त्रिपुरम् is neuter, and the word could as easily have been त्रिपुरी
uncheckedMallinātha, Sañjīvinī, who gives the samāhāra, cites 2.1.51 by its opening words and argues the gender from the पात्रादि class; the texts of 2.1.51, 2.4.17 and 4.1.21 from skt-mcp. The पात्रादि list itself is a gaṇa, and skt-data carries no gaṇapāṭha, so what the class contains cannot be checked from the tools“trayāṇāṃ purāṇāṃ samāhārastripuram || “taddhitārthottarapada-” iti samāsaḥ | pātrāditvānnapuṃsakatvam || tasya vijayo gīyate”