The term Tretagni in Hinduism refers to the group of three fires, namely, grihapatyagni, dakshin agni and ahavaniyagni, used for making oblations in a Srauta Karma. The basins used for these three fires are circular, semi-circular and square in shape, respectively.
It is quite interesting to note the way the dimensions of these basins and the altars are specified in Sulba Sutras.
The texts give the dimensions of the basin for ahavaniyagni and merely mention that the basins for the other two fires have to be constructed in such a way that the area of the basin for ahavaniyagni is the same as the area of the other two basins.
Thus, the problem of finding the radius for the basins of grihapayagni and ahavaniyagni is left to the performers of the sacrifice. The space in between in the grihapayagni and ahavaniyagni in the east-west direction is referred to as vedi, and the darbha (holy grass) is spread over it.
A schematic sketch of the basic structure of the vedi (altar), used for performing srauta karmas and the positions of the different priests are indicated.
An ahitagni is supposed to preserve the fire in this house continuously, from the day he performs the adhanakarma till he breathes his last. Usually this is preserved in this agnihotrasala, the place where the daily ritual Agnihotra is performed in the altar meant for grihapatyagni. The term griha refers to house and pati refers to the head. Hence the term grihapatya refers to the head of the house. An ahitagni considers the grhapatyagni to be the presiding deity and the head of the house and preserves it continuously with great care and veneration.
When specific rituals or yajnas such as Agnihotra, Ishti, etc., are to be performed, the fire from the grihapatyagni will be transferred to other basins, too. Once the ritual or yajna is over, the other fires are allowed to cool down, whereas the grihapatyagni will still be maintained. In fact, it is this grihapatyagni which will be used for the cremation of an ahitagni on his death.
In all the yajnas, pradhanahuti (the main offerings) are done in ahavaniyagni. For instance, in a soma yajna, the offering of soma juice, which is considered the pradhanahuti, will be made in ahavaniyagni. In Agnihotra, which is a daily obligatory ritual, the two oblations to the deities Surya and Prajapati are the main offerings, and they are done in ahavaniyagni.
The other two fires, namely, grahapatyagni and dakshinagni are reserved for subordinate offerings known as angahutis. For example, a class of subordinate offerings called patnisamyaja are done in grahapatyagni. Another class referred to as prayascittahutayah are done in dakshinagni. In a particular ritual, dakshinagni is used for cooking food that is to be offered to the priests. This food which is to be cooked is named anvaharya, and hence dakshinagni is also referred to as anvaharya pacana in Vedas.
Notes taken from Encyclopedia of Hinduism Volume X page 30 - IHRF
Notes taken from Encyclopedia of Hinduism Volume X page 30 - IHRF