Manava Dharmashastra 2.231

2.231

kenacittsāmānyenāyaṃ pitrādīnāṃ gārhapatyādivyapadeśaḥ | sā'gnitretā ādhānāgnitretā yā garīyasī mahāphalā | trāṇaṃ trāṇārthamitā prāptā tretā iti śabdavyutpattiḥ |

The Father has been declared to be the Gārhapatya Fire, the Mother the Dakṣiṇa Fire, and the Preceptor the Āhavanīya fire; and this Triad of fires is highly important.—(231)

Medhātithi’s commentary (manubhāṣya):
The father, etc., have been called ‘gārhapatya’ and the rest by reason of some sort of resemblance.

‘This Triad of Fires’—i.e., the three sacrificial Fires—‘is highly important,’—i.e., conducive to great results.

The word ‘treta’ (Triad) is etymologically analysed as ‘trāṇam itā’ which means ‘got up for the purposes of protection.’—(231)