All Shrauta yajnas or yagas are associated with Tretagni (Samskrit : त्रेताग्नयः) the three Agni's, and one of them Garhapatya agni is also called Shrautagni. However, not all three fires are lighted and maintained at all times. त्रेताग्नयः the three fires are kindled in performance of yagas. Shrautayajnas require the kindling of all three fires.
The earliest instances that Yajnas were conducted with the installation of three fires appears clear from the three mantras of Rig veda as follows
अग्ने देवाँ इहा वह सादया योनिषु त्रिषु । परि भूष पिब ऋतुना ॥४॥ (Rig. Veda. 1.15.4)
agne devām̐ ihā vaha sādayā yoniṣu triṣu । pari bhūṣa piba r̥tunā ॥4॥
Meaning : Oh Agni! Bring the devatas here and seat them in the three designated places. Decorate them and drink somarasa according to the seasons. Pt. Satvalekar in his Rig veda Subodh Bhashya explains the place of Agni in different parts of the body, the head, heart and the lower place of the body being the three important places.
आ वक्षि देवाँ इह विप्र यक्षि चोशन् होतर्नि षदा योनिषु त्रिषु ... ॥४॥ (Rig. Veda. 2.36.4)
ā vakṣi devām̐ iha vipra yakṣi cośan hotarni ṣadā yoniṣu triṣu ... ॥4॥
Summary : Oh ! Agni, who invites devatas for the yajna, and pray to them. Agni, who presides over the yajnas, shine forth in the three places (3 lokas according to Pt. Satvalekar).
यज्ञस्य केतुं प्रथमं पुरोहितमग्निं नरस्त्रिषधस्थे समीधिरे ...॥२॥ (Rig. Veda. 5.11.2) 
yajñasya ketuṁ prathamaṁ purohitamagniṁ narastriṣadhasthe samīdhire ...॥2॥
Summary : The sign of yajna, the earliest priest who brings all the other devatas and seats them along with him, this Agni is kindled by men in three places.
आहिताग्निन् ॥ Ahitagni
One who worships the three Agnis is called a "Srauti" and if he worships the Shrauta (garhapatyagni) and grhya agni (aupasanagni), he is called an Ahitagni.
- The section in the Shrauta-sutras dealing with the offerings made in the three fires is called Tretagni-kanda in Apastamba Shrauta sutras.
- The grihastha who never performed any Haviryajna and never studied Vedas, and whose father or forefathers never performed yaga are not eligible to perform some yagas/yajnas. Example : Somayaga
- Eligibility for a grihastha to perform other yagas or yajnas will be obtained once he regularly performs Aupaasana at his house on a daily basis and after he learns the relevant vaidika procedures to conduct them . For example: Agnisthoma is the first of the 5 Jyotisthomas or Somayaga. Performance of haviryajna and some isthis is a prerequisite for conducting a Somayaga.
The tradition of ‘Aahitagnis’ and ‘Shrauta priests’ continues in South India and they perform Vedic rituals as and when possible. These priests inherited the tradition of Vedic ritual practices from their ancestors and perform Vedic rituals even today.
त्रेताग्निः ॥ Tretagni
Shrauta yajnas require the kindling of Tretagni or three Agnis (Singular : त्रेताग्निः Plural : त्रेताग्नयः) into which ahutis are offered.
- गार्हपत्याग्निः ॥ Garhapatyaagni
- आहवनीयाग्निः॥ Aahvaniya agni
- दक्षिणाग्निः ॥ Dakshinagni
Asvalayana Shrautasutra mentions the following about the three agnis in the context of yajna kunda पर्युक्षण (sprinkling of water). Before kindling of fire in the kundas sprinkling of water should be done by hand.
दक्षिणं त्वेव प्रथमं विज्ञायते पिता वा एषोऽग्नीनां यद्दक्षिणः पुत्रो गार्हपत्यः पौत्र आहवनीयस्तस्मादेवं पर्युक्षेत् १३
dakṣiṇaṁ tveva prathamaṁ vijñāyate pitā vā eṣo'gnīnāṁ yaddakṣiṇaḥ putro gārhapatyaḥ pautra āhavanīyastasmādevaṁ paryukṣet 13
Meaning : It is to be known that the dakshina fire should be done first, because dakshina fire is the father of all fires, garhapatya is the son and aahvaniya is the grandson. So water पर्युक्षण water should be given in this way. A few important features of the three agnis are summarized below.
गार्हपत्याग्निः ॥ Garhapatyagni
गार्हपत्येन सन्त्य ऋतुना यज्ञनीरसि । देवान् देवयते यज ॥१२॥ (Rig. Veda. 1.15.12) 
gārhapatyena santya r̥tunā yajñanīrasi । devān devayate yaja ॥12॥
Summary: Agni, who fulfills the desires, by presiding over the Garhapatya agni, he guides the yajnas and grants heavenly enjoyments to the yajamana. "Garhapatya" Agni is thus expressly mentioned in Rig Veda.
A few aspects about of गार्हपत्याग्निः ॥ Garhapatya agni are as follows 
- It is kindled after marriage, and is be maintained and worshipped for the rest of the life by the grihastha along with his wife.
- It is ceremonially established with the elaborate vedic procedure involving recitation of specific mantras during the process called Agnyadhana. The Aupasana Agni lit at the time of the wedding is passed on from the father to the son. It is divided into two during the Agnyadhana process. One part becomes the Grhyagni (Smartagni, Aupasanagni) and the other becomes the Shrautagni called Garhapatya.
- It is the only Agni of the three Agnis that is maintained continuously.
- It is placed to the west of the yajnavedi and placed in circular shaped kunda in Shrauta yajnas.
- It is rekindled by a special process involving the Prayaschitta mantras called "Punaradhana" if it gets extinguished for any reason.
- It is used in nityakarmas (Agnihotram) as well as in other Shrauta Yajnas.
- It is the source of other two Agnis, Aahvaniya and Dakshinagni.
- It is used to cook aahutidravyas and for heating by reciting some mantras. No specific ahutis are performed in this Agni.
- Only a sapatnika yajamani (grihastha) of the three varnas is prescribed to tend this Agni according to some sources, while others mention that persons of all varnas may do so.
आहवनीयाग्निः ॥ Ahvaniyagni
Some salient aspects about आहवनीयाग्निः ॥ Ahvaniya agni are as follows
- Aahvaniya agni is established from the Garhapatya agni along with the recitation of specific mantras.
- It is not maintained continuously everyday and is considered as Laukikagni at the end of the day at the conclusion of specific purpose for which it is invoked.
- It is placed to the east of the yajnavedi, in a square shaped kunda.
- It is used to offer aahutis to the devatas during Shrauta yajnas.
दक्षिणाग्निः ॥ Dakshinagni
Salient aspects about दक्षिणाग्निः ॥ Dakshinaagni is as follows
- Dakshinaagni is established from the Garhapatya agni along with the recitation of specific mantras during yajna karmas.
- It is not maintained continuously everyday and is considered as Laukikagni at the end of the specific purpose for which it is invoked.
- It is placed to the south of the yajnavedi, in a half circle shaped kunda.
- It is used to offer aahutis to the pitrudevatas or forefathers only.
त्रेताग्नेः उपासनावैशिष्यम् ॥ Tretagni Upasana
Manusmriti extols the greatness of Tretagni Upasana.
त एव हि त्रयो लोकास्त एव त्रय आश्रमाः । त एव हि त्रयो वेदास्त एवोक्तास्त्रयोऽग्नयः । । २.२३०
ta eva hi trayo lokāsta eva traya āśramāḥ । ta eva hi trayo vedāsta evoktāstrayo'gnayaḥ । । 2.230
Meaning : For they are declared to be the three worlds, they are the three ashramas, they the three Vedas and the three fires.
पिता वै गार्हपत्योऽग्निर्माताग्निर्दक्षिणाः स्मृताः । गुरुराहवनीयस्तु साग्नित्रेता गरीयसी ।। २३१ (Manu. Smri. 2.231)
pitā vai gārhapatyo'gnirmātāgnirdakṣiṇāḥ smr̥tāḥ । gururāhavanīyastu sāgnitretā garīyasī ।। 231
Meaning : The father is stated to be the Garhapatya fire, the mother the Dakshinagni, the teacher the Ahavaniya fire; these three fires are most venerable.
त्रिष्वप्रमाद्यन्नेतेषु त्रील्लोलोकान् विजयेद्गृही । दीप्यमानः स्ववपुषाः देववद्यिवि मोदते ।। २३२ (Manu. Smri. 2.232)
triṣvapramādyanneteṣu trīllolokān vijayedgr̥hī । dīpyamānaḥ svavapuṣāḥ devavadyivi modate ।। 232
Meaning : He who does not neglect those three, (even after he has become) a householder, will conquer the three worlds and glows with radiance, enjoying heavenly pleasures.
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